This is easily the simplest and te easiest to make ice-based self-bondage time-lock. As you can see on the picture, you will probably need to buy only one thing - as metal ring. I assume you have already got the the rest (ice, cotton bag from a supermarket, rope)

This is an anchor lock. Put some ice blocks into the bag, bind the metal ring with a rope to, say, your legs (bound, of course), thread the other part of the bag through the ring and tie it down to a stationary object. Drop a knife outside of your reach and bind your arms.

You will not be able to get to the knife until the ice melts and you squeeze the water out of the bag.

You can use any suitable sack-like thing instead of a bag. For example, a sock.

This method is suitable for self-bondage sessions in hotels with ice-machines or during cold winter when ice and snow available everywhere. See this post.