If you already have clean piss, put it in one of those guaranteed not to leak baggies, let as much air out as possible and tape it to the inside of your leg.
You could always buy the synthetic piss, jock it all day and it'll stay at the temp you need...plus it comes with a heat wrap that is good for several hours.
Or, buy the drink that they sell at headshops and such. They work like a charm. You will get a NEGATIVE result and not an INCONCLUSIVE.
Depending on what is your DOC, you don't need as much time as you would suspect. I've known heavy smokers to drink a ton of water and take some niacin and be clean for a test in a few days.