I just thought of the best release mechanism P:.. You have a little safe that can fit a key, small scissor etc. You insert the object, and after a certain period of time the safe door opens, and you can unlock/cut loose.
Anybody knows if this is purchasable somewhere 😋?
I don't like to trust any electrical or mechanical devices, as they can fail. For electrical devices, I would prefer those that release me instantly on power failure and where all components rely on the same power source. Like this, a temporary power outage can't make the session inbearably long (timer...). And unless a mechanical device falls apart, it will almost certainly never ever release you if it fails. On the other hand, a really well designed mechanical device could be fairly safe. I think, the lest reliable devices are computer based because even a perfect software (which would be quite some achievement) does not prevent a computer to crash, freeze or fail because of power failure and then, whatever should happen physically, won't happen.
The best method in my opinion is melting ice. Unfortunately, the timing is not well predictable, but logging a few experiments under various circumstances should help.