It sounds like you're going to be able to do this in private. If you can bring water to add, why not just use clean urine? Ask somebody that hasn't been toking, not taking prescription narcotics or anything illegal, etc., to pee in a small waterproof bottle - like a vitamin container that's been washed. You won't need much. I did that, years ago, and I passed with no trouble.

Some places test the temperature of the urine, some watch you, but there are devices available, worn under the clothes, to beat these methods. I once saw ads for freeze dried mail-order urine that was guaranteed clean.