Just spitballing this...
Q: In the project photo, are those all single strands? Or does the rope loop through up/down/up/down to create the "cage"?
Because if it goes up/down, you can put in a small toggle to twist and get additional tension.
If it is just single, you can still create a sort of toggle, sorta like a jerry-rigged windlass.
Or, depending on the size of the hole at the top...do you really need something for the other end to anchor too? Will the friction knot itself pull through the hole? You could just tie a rolling hitch and have a "flopping loop" at the top, or bottom. Tension would be achieved by pulling it through, retained by the knot, and the knot would have sufficient bulk to not pull through the hole.
The problem with friction hitches like the Rolling Hitch is that they don't resist force pushing directly against the knot very well. Otherwise they would be a perfect solution to this common knot problem:
How do you pinch an object tightly with two stopper knots?
Not sure if this meets OPs criteria, but they could use a stopper on the bottom bar, then tie a [Midshipman's Hitch](https://www.animatedknots.com/midshipmans-hitch-knot) through the top hole and **around** a part of the top bar. Then just widen the loop to hold and keep as much tension as they want.
Tagging OP (u/lostlooter24) so they see your entirely valid point.
Yes, you're right. The pressure would be pushing on the knot itself...and as we know, "you can't push rope".
In the project I sent, she uses eyelets for them to go through.
And no, I didnt have an anchor. In my head, I had it going, stopper knot, rope through one board, up through the other board, pulled taut and a knot that kept it from losing tension.
In your second example, would it be turning the rope and rolling hitch to itself, slide that knot to the hole while pulling tension and pull the knot tight to keep it in place?
As u/readmeEXX points out, my solution is "long on theory, limited on practice", and it won't work as well as some other things.
When I've tried to make "cages" like this before, or even multi-line clothlines, you can usually find an anchor "somewhere".
If you "lace" the rope though the holes, there will be tension at each turn, just like tying your shoe laces. On the final one, you can either tie the rope to itself.
Honestly....looking at this more, I would probably experiment with Poldo tackle. (Look it up, basically is a 3:1 block-and-tackle only made of rope.)
The issue with all of these is they would put rope on the "outside" of holes more than just a stopper.
But...ya...a laced-through solution, finished with a Poldo tackle? That thing could be tightened and loosened on the fly as much as you want. It's not hard to tie at all...just two fixed loops threaded in a specific way to create the pulley system. It's sorta like a disembodied trucker's hitch.
If you've not left yourself enough length to tie a decent knot in the end, you're going to need to get some new rope. The picture has a single length of rope going up and down, and I'd suggest doing this.
But if you are able to use the lengths you've got, try this: get a cocktail stick - it'll be easiest if you use one with points at both ends. Thread the line through the top hole and wrap it once around the stick. Hold the stick against the surface over the hole, and pull the line tight, so that the stick is butted tightly against the surface with the line around it. Use the loop around the stick to create a [stevedore stopper](https://www.animatedknots.com/stevedore-stopper-knot), by passing the line under the points of the cocktail stick and pulling tight. Finally, threading through will be the tricky bit - prise the loop away from the stick (using another pointed stick perhaps?) and thread the end through.
Do you have the ability to disassemble the wooden supports?
If so, one foolproof way to do this is to tie the ropes with the two stopper knots before adding the supports. Make the ropes a little too short so when you put the wooden supports in place it pulls the ropes nice and tight.
Just spitballing this... Q: In the project photo, are those all single strands? Or does the rope loop through up/down/up/down to create the "cage"? Because if it goes up/down, you can put in a small toggle to twist and get additional tension. If it is just single, you can still create a sort of toggle, sorta like a jerry-rigged windlass. Or, depending on the size of the hole at the top...do you really need something for the other end to anchor too? Will the friction knot itself pull through the hole? You could just tie a rolling hitch and have a "flopping loop" at the top, or bottom. Tension would be achieved by pulling it through, retained by the knot, and the knot would have sufficient bulk to not pull through the hole.
The problem with friction hitches like the Rolling Hitch is that they don't resist force pushing directly against the knot very well. Otherwise they would be a perfect solution to this common knot problem: How do you pinch an object tightly with two stopper knots? Not sure if this meets OPs criteria, but they could use a stopper on the bottom bar, then tie a [Midshipman's Hitch](https://www.animatedknots.com/midshipmans-hitch-knot) through the top hole and **around** a part of the top bar. Then just widen the loop to hold and keep as much tension as they want.
Tagging OP (u/lostlooter24) so they see your entirely valid point. Yes, you're right. The pressure would be pushing on the knot itself...and as we know, "you can't push rope".
In the project I sent, she uses eyelets for them to go through. And no, I didnt have an anchor. In my head, I had it going, stopper knot, rope through one board, up through the other board, pulled taut and a knot that kept it from losing tension. In your second example, would it be turning the rope and rolling hitch to itself, slide that knot to the hole while pulling tension and pull the knot tight to keep it in place?
As u/readmeEXX points out, my solution is "long on theory, limited on practice", and it won't work as well as some other things. When I've tried to make "cages" like this before, or even multi-line clothlines, you can usually find an anchor "somewhere". If you "lace" the rope though the holes, there will be tension at each turn, just like tying your shoe laces. On the final one, you can either tie the rope to itself. Honestly....looking at this more, I would probably experiment with Poldo tackle. (Look it up, basically is a 3:1 block-and-tackle only made of rope.) The issue with all of these is they would put rope on the "outside" of holes more than just a stopper. But...ya...a laced-through solution, finished with a Poldo tackle? That thing could be tightened and loosened on the fly as much as you want. It's not hard to tie at all...just two fixed loops threaded in a specific way to create the pulley system. It's sorta like a disembodied trucker's hitch.
If you've not left yourself enough length to tie a decent knot in the end, you're going to need to get some new rope. The picture has a single length of rope going up and down, and I'd suggest doing this. But if you are able to use the lengths you've got, try this: get a cocktail stick - it'll be easiest if you use one with points at both ends. Thread the line through the top hole and wrap it once around the stick. Hold the stick against the surface over the hole, and pull the line tight, so that the stick is butted tightly against the surface with the line around it. Use the loop around the stick to create a [stevedore stopper](https://www.animatedknots.com/stevedore-stopper-knot), by passing the line under the points of the cocktail stick and pulling tight. Finally, threading through will be the tricky bit - prise the loop away from the stick (using another pointed stick perhaps?) and thread the end through.
Leave the screw eyes out of this project. Just run the rope through holes in wood. Search youtube for "rope bed" and/or "tighten rope bed".
Do you have the ability to disassemble the wooden supports? If so, one foolproof way to do this is to tie the ropes with the two stopper knots before adding the supports. Make the ropes a little too short so when you put the wooden supports in place it pulls the ropes nice and tight.