I don't know about all the other ideas you've been told but I substitute all the time and when you say your not sure if you can, no matter where you take a employment test you go in the bathroom alone and come out with a cup of piss.

Search for quick fix and substitution here and you will heat it up, attach a heating pad, put it in the pouch of your fruit of the looms, go in the bathroom and pour it in the cup and with a search and practice you will know how to check temperature and it could not be easier.