Here is what I did:

I heated up the bottle to around 100, it was on accident but I figured it would cool down to proper temp from there.

I put the bottle in the taint area, had it nice and secure with two tighty whities under my normal boxers, and let it stay there. It cooled down to around 97 or so, and stayed there due to my legs and crotch sharing body heat with it. When it came time to do the piss, I pulled out the bottle, checked the temp on the side, which is important since if the temp is off you need to say you cant piss and can you have 10 minutes and some water so that you can let your body warm up the bottle some more, and then poured it in, put the bottle back, and was done. Its that easy. =)