Basic corner clamp for holding a boxes sides in position when gluing it.

Print 4 of these, put some string/wire/fishing-line around them, use a simple rod as a tourniquet to tighten the line and you've got yourself a box being held at right-angles.

These parts SHOULD print without supports but I haven't experimented with different printers yet. I'd recommend thick walls (3+ layers) and/or internal structure to make these strong.

OpenSCAD file included so you can customise (or customize :) ) the size to your needs.