Just for fun, something to keep that special someone in your life close at hand;-) Printed in PLA at 40% infill they are quite sturdy- although breakable in a pinch (a requirement in anything that encircles the neck). The rings work well with paracord.The Large Collar has a 14cm internal diameter. The Large Manacle's internal dimensions are 5.6 cm x 6.5 cm.The Small Collar has a 12cm internal diameter. The Small Manacle's internal dimensions are 4.9 cm x 6 cm.They use two pieces of hardware from McMaster-Carr. Based on the recommendation of the iHeartRobotics blog ( http://www.iheartrobotics.com/2012/05/fastners-for-3d-printing-threaded.html -thanks!) we went with a brass insert:http://www.mcmaster.com/#94180A353 "Metric Brass Heat-Set Insert for Plastics, Tapered, M4-.7 Internal Thread, 7.9MM Length"…and a screw:http://www.mcmaster.com/#91292A130 "Metric 18-8 Stainless Steel Socket Head Cap Screw, M4 Thread, 30MM Length"You can get the special heated tool for the insert but I was able to make do with a regular soldering iron and a bit of patience.Shown (modeled by the lovely M. Zhao) is the small size, not being sexist or trying to oppress anyone- I honestly have more use for the large/guys size but I need to find someone with a printer with a large enough bed so we can make it.