This is tried and proven by someone i know...cough...

Buy a synthetic urine kit. While driving on the way to the lab turn your car heater on HIGH and hold the bottle to the heater vent for a few minutes (dont worry, cant over heat it with this). By the time it has reached max temp it falls at around 102 degrees (my experience). This will give you about 15 or 20 minutes to fill out the forms they hand you, and dump the stuff in the bottle while still keeping a good temp.

Its very easy to carry in.... look at all the places people hide their stuff on the show cops, you can never see it until they poke around.