This thread was incredibly helpful, so I wanted to add my experience at Quest Diagnostics.
I had to pass a pre employment drug screen -- and I mean, I really needed to pass, this job was...is...everything to me. I knew from research that I had to get synthetic urine with urea and uric acid. I also knew the temperature had to be between 90 and 100 degrees when I turned my sample in to the lab tech.
So, on the morning of my test, I popped the bottle into the microwave just like the instructions directed (don't forget to shake the bottle). After ten seconds, it was 98 degrees, and I didn't want it to fall any further on the drive over to the lab, so I popped it in for another five seconds and it was registering about 105 degrees on my digital thermometer. I replaced the cap (I read that the squirt cap can sometimes make noise, so I used the cap originally on the bottle and just threw away the other) and strapped the heat pack to it. I secured the bottle with the heat pack to my leg using an ace bandage and wore bell bottom jeans. It was secure and no one could detect anything.
When I got to Quest, I made small talk with the lab tech, and she gave me the standard spiel: fill the cup, don't flush and don't wash your hands. I had three minutes to give my sample, which made me nervous, but I just reminded myself how long three minutes actually is and got down to work. I peed a little into the toilet and then rolled up the leg of my jeans, unwound the ace bandage and poured the sample into the cup. It was 98 degrees in the XStream bottle and when I poured into the sample cup, the temperature strip on it read 96 degrees. Perfect. I reattached the empty bottle with the ace bandage, opened the door, the lab checked the temperature and voila! It was all done.
I easily passed. It wasn't my first choice, and I've since given up entirely (for the time being -- like I said, this job is everything to me, and I want to do my very best), but it can easily be done. Be cool, be confident, practice your methodology and you're good to go.
