In your circumstances I would recommend a pouch inside your pants that will hold the synthetic urine and an unactivated heat pad. If you get the call reach in your pocket and squeeze the heat pad and activate it. It usually takes 30 to 40 minutes for the pad to get up to temp. Again practice to get the routine down.
Also, cost these assholes money. its the only thing they understand. After 3 hours of waiting for you to pee they have to release you and reschedule you for another day. Cost them as much money as you can by stalling and getting paid for sitting on your ass. This is the only thing that will force change.