Parametric sized collar based on the excellent looks of the Swedish Collar. The rear hinge has a locking pin which is inserted into place, and the front locks closed using a magnetic pin - it's sized for a 1/8x 1 neodymium magnet rod. Once dropped in the only way to remove the rod and open the collar is with a similarly strong neodymium magnet. I bought mine from a site called MAGCRAFT, but there are multiple vendors.

Additionally you can print the second ring and set it up on your build plate so it's directly in front of the half-ring built into the collar, and lean it back 30 degrees. It can be printed in place through the half-ring, assuming your build plate is sticky enough. During my prototyping I did it once successfully and twice unsuccessfully, but each failure only ruined that extra ring, it did not harm the collar.

The provided stl is sized for a 14" circumference neck. The fusion file can be used to generate a correctly sized collar for you.

To size the collar, measure with a string or flexible measuring tape the wearer's neck, directly over the adam's apple area. Divide the measurement by pi and input into the NeckDiameter user parameter in the fusion file, then export the parts you want. The collar is meant to be fairly snug - if you add a little spacing it really shouldn't be much at all, 1/2" at most. I added none for the example collar shown. The collar has a bevel on the lower edge to help the collar fit comfortably.