If you can sneak in water then you can sneak in a clean donor's urine sample or buy synthetic urine online this weekend. It isn't that hard to get the temp right. Just get a clean sample and practice a couple of times heating up water in the covert bottle of choice, securing it to your body, then retesting the temp to be within 90-100 degrees. Your body can easily keep a sample at the right temperature if it starts off in this range or after a few hours it can bring a slightly cooler sample up to temperature.