You could still sub. Get the synthetic urine and heat and do everything as you normally would but have a piece of clear tubing stuck on the end of the bottle and just use that. Make sure the tube is attached securely to the bottle and make the hose about 6 inches long. As long as the end of the tube is higher than the level of the fluid is it won't come out. You could even put a simple little plug on the end to make sure it doesn't leak. Then just practice with it till you got your technique down.