To create the clamp print three copies of corner_clamp_plain, and one copy each of corner_clamp_ratchet and corner_clamp_key.  All three files do better with “support everywhere” but support everywhere is necessary only for corner_clamp_ratchet.  I use ordinary PLA, finding its strength and rigidity adequate.  Finish the clamp with a length of 3 mm paracord and a rod about an inch and a half in length (40mm), up to one-eighth inch in diameter (3mm).  Barbecue skewers, cue-tips, filament, and coat-hanger wire are fine sources for a rod.  The keys in the photo are held using cue-tips.  The key fits into the funny shaped hole in corner_clamp_ratchet.  Feed the paracord through all the holes and channels (troughs).  The hole in the key's shaft is parallel to the grip so feeding the cord through the hole is easy by orienting the grip in line with the channel.

The size and light-weight of this design is suited to making small boxes, as small as 3-¼ by 2 inches or 83 by 51 mm. 

In gluing a box, start by adjusting the length of cord.  Undo its knot then spread or shrink spacing just a bit greater than the box dimensions.  Tie a knot leaving as little slack as you can. Turning the key will cinch up the clamps.  When it's as tight as you like, insert the rod into a hole in the key's grip through to a hole in the ratchet housing.  The odd number (7) of holes used in their spacing – one was removed – sets them at intervals of 51 degrees.  But because the number is odd, when the key's hole is aligned with one, the hole at the opposite end of the key is over a space between two holes.  Effectively that means the resolution is closer to 26 degrees than 51.  The point is: if you would like to cinch only a little tighter, but not as much as moving to the next hole, move the rod to the key's opposite hole for a half-step of tightness.  Thanks to the accuracy of 3D printers, the corners have 90 degree faces and may be all you need to bring the box into square.

In addition to the stl files, OpenSCAD source files are included should you want to modify the dimensions or otherwise alter the design.  I would love to hear of the improvements you've made.