I recommend to use nade74’s remix using a metal spring, (https://www.thingiverse.com/thing:4785326) since the spring tends to deform overtime, causing the cuffs to not lock anymore.

This is my take on Thumbcuffs, it's 100% 3D printable, and no additional hardware is required, even glue is optional! You need a vise to assemble it though. It holds itself together with dove joints. And the clip won't come of even if you didn't glue it and are pulling on it with pliers, and it even holds if the hinge snaps off due to the included groove. (I tested it) So it is very Sturdy.

You probably need to resize it to fit your toes.

Both sides are unlockable through only one key hole, and the keyhole has a little sinking to make picking harder, however even with a standart paperclip you can pick it while your tumbs are in there with relative ease. But i wanted to make sure that the key doesn't snap accidentaly.

Use a flexible material like Hard TPU for the ratchet spring, and a rigit like PLA for the rest. Your printer needs to be able to print 45 degree overhangs without a problem. Print the Key lying flat with supports!!