I've never submitted an actual piss sample at any point.
I still have the prosthetic dick from probation as a minor.
For pre-employment I warm it to the right temperature in a sous vide bath but you can use a pot of water on a burner just fine. The little bottle/reservoir thing I have from a Sub Solution kit has one of those strips with colored dots that appear at certain temperatures to make sure you're in the range. Then I just hide it in my underwear with an adhesive heating pad or strap it to my leg and go "piss".