If you're going to substitute, hot hands don't work- they're not strong enough to keep the temp up. Try Thermacare pads- you can find them in the pain relief section at any Rite Aid. I would try heating with just your body temp though. Easier to ensure the temp stays constant. Buy extras of everything and practice it several different ways to make sure you get the temp of the sample exactly right. It lessens the nervousness on the day of the test.