How to stay ready at work for random drug tests?

I work for a non-profit organization that is government funded and am subject to random drug tests. They have a zero-tolerance policy. When a drug test happens, it is on site, and HR comes and lets you know the drug testing people are here, and to head downstairs to take the test. Is there anyway to be able to heat up synthetic urine in a very short amount of time without being obvious? I do have access to a microwave, but that would be a little weird using the microwave right before a drug test.. any ideas? I have a desk I could store it in, just unsure of how to get it to temp fast and sneaky.