UKC

I've got a square wooden frame that I want to stabilise by having two diagonal ropes. I have an eyelet at each corner. What knot do I want so that I can tie on it and then yank on it to add (quite a bit of) tension to the rope?

I tried a taut line hitch but I couldn't really get enough tension on it.

I would use a truckers hitch.

Ratchet straps

Form loops (whatever knot you prefer), tension them with a Spanish windlass, lock it off to the other diagonal and visa versa.

Or yeah, ratchet straps.

Jk

How about getting or making some tent guy line tensioner jobbies?

It you really want tight, make a complete loop between corners. Put a stick through the loop and wind away.

thanks all, was hoping there was some magical knot that my google foo was too poor to find.

+1 to the suggestion of ratchet straps.

Here's another idea you could try though - three ropes.  Tie two along each of the long sides using a trucker's hitch to tighten them if necessary (or perhaps one big loop all the way around the outside). 

Then use a third to pull the centres together, you could wrap it around them a couple of times to give you a bit of mechanical advantage and progress capture.

I'm not guaranteeing it'll work, guessing a bit.  Might need a bit of trial and error to get the tension in the sides right so that you can just barely bring the centres together.

Hang on a sec..  here's a sketch..

Oh - it just occurred to me that you could probably modify that idea a little bit to work very well with the Spanish windlass suggested above.  Two ropes on the long sides, and make it so you can just about stretch them both into the middle, where you pop a stick in between them and wind it up tight from there.

Can you thread an Italian hitch on one end and use a jumar and a pulley to give you a 3 to 1. Then just tie off the Italian? 

The thing is, knots lock off but they won't introduce tension. You could use a prussik or a barrel knot to capture progress, but you're never going to get the sort of tension you would get without some sort of mechanical advantage aka ratchets or pulleys etc. The stick idea someone suggested is quite clever, you could tie off the stick afterwards. 

The truckers hitch, that has been suggested, will introduce tension. 

 Using a tarbuck knot can be good to lock off, especially  if used with extra threading to gain mechanical advantage, with or without pulleys.

Climbing ropes will lose tension as the rope relaxes with time. Better to use ropes not made out of nylon.

How big is the frame and what are the eyelets made of - how much friction is in them?

Depending on that answer, I'd either tie the rope ends into the corners and use a truckers hitch to tension, or just use a running loop and a truckers hitch.

If you really want tension, then I'd 2nd the suggestion of roof rack straps - small ratchet ones of those are quite cheap and also won't stretch much.

I agree with those that have suggested the Trucker's hitch. Simple and effective. You can even tie it so it self locks as you tension the line. Lots of examples on YouTube.

Obviously dismissing ratchet straps straight away as too sensible.

Wide_Mouth_frog is the only answer so far that makes any effort to introduce climbing equipment in any quantity. Surely for this forum points should be awarded for each krab used, double your points for including any more complex equipment which can be doubled again if one piece of equipment interacts with another, and the level of overcomplexity is the tie-breaker if needed. 

I'm going to propose: using a couple of slings in a very very wide angled sliding X, top and bottom (standard krabs) and applying tension between these with a bit of rope and a grigri (belay krab) to capture progress but tensioned with a hand jammer and pulley (Oval krab essential)

This is obviously the simplest solution. 

Do I get any nerd points for pointing out that, if not using ratchet straps, the OP doesn't need a knot. What they need is a hitch? 

I think points should be deducted for saying this.

Drill the legs, insert 3/4" threaded rods with big washers. Tighten nuts with big spanner.

Points deducted for using the Johnson/Farage favoured historical measurement system surely you meant 3/8 (slightly small 10mm) not 3/4? (19mm).

And remember the old agricultural adage. It doesn't matter if the threads match, as long as  your spanner is long enough!