Go to a dollar store you can buy cheap heating pads there.
I don't activate the pad till I've been called for a random. I just keep the QF and pad in my pocket till I my random. Then if I don't have access to a microwave I'll activate the pad and move both to my crotch. You don't have to be in a hurry to take the UA. Most people don't pee on command so just give the QF time to heat up.