A reverse fiddle designed to hold a persons hands behind their head instead of in front of it. Designed it a while ago, printed a prototype, and then moved on to other projects. Figured I'd upload it here, it's a relatively simple design.

Normal size has a 5.5" ID neck, and the wrist ellipse IDs are 2.0" x 2.5". Large size has a 6.5" ID neck, and the wrist ellipse IDs are 2.5" x 3.0".

Printable hardware files are included. The captive pin has grooves to snap in to place, the lockable pin can be locked with any small padlock (1/8" or ~13mm dia shackle, or smaller). The holes are sized for 1/4" hardware, so if you'd prefer to use other 1/4" pins (like a cotter pin) you can. All you need is a padlock with a 1/4" shackle to lock the thing on someone (though, you could print another pin for this too, if you really wanted to). If you have trouble with the pins fitting, specifically the captive one, you can scale them slightly on the X and Y (assuming you've already calibrated your steps and flow).