I like the kneeling idea. How about a bag of frozen lima beans on the head? It will be cold, drip on their shoulders, fall off occasionally and you can feed them to the sub later if they fail.

Since we were talking about ice: Take a pair of butterfly clamps and freeze the center of the chain in a large plastic cup of water. Sub kneels and the clamps are attached to the nipples. (frozen clamps - yikes). Now you have a large ice-cube. The sub can lean forward so it does not touch them, or lean back so the ice rests on their stomach and melts faster to reduce the weight sooner.