how about you fill a condom or two with someone elses piss, tie themoff, then stick them in a jock/pocket. next take one of those handwarmers u get at a ski shop and stick that in there to. Then it'll be plenty warm.

also chekc these out:
http://www.cleartest.com/products/syntheticurine.html
http://www.cleartest.com/products/bakenshake.html
http://www.cleartest.com/products/urineadditives.html

some of them can get a little pricey, but I know a guy who used the detox bag and it worked for him...

also you could just list on the sheet for over the counter meds: aspirin,motrin, and ibiprofen. they all have been known to get false positives.