FunToies Timecuffs Deluxe TC 5 Hour Handcuffs
Fill the chamber with water, push the cable with the key at it's end into the chamber till the cork is snug. Freeze it over night. Put them on. Can't take them off till the ice melts - 5 hours according to the manufacturer (see below). Using hot water on the chamber helps little due to the rather good insulation it offers. The cuffs are Smith & Wesson 104 Handcuffs which prevents "casual" picking with a standard handcuff key.
A foolproof timed release restraint device is hard to design. This is as close as anything I have seen.
It is marketed for novelty purposes only for use by singles or couples.
The device works, to a point. After an hour and a half, the ice melts enough that the cork can be pulled out a bit. When this is done, simply letting hot water flow into the chamber will melt the rest in seconds.
Another potential down side is that if one were to fall asleep wearing these, once the ice melts and the cork pulls out, the liquid water would likely splash on ones body.