IMO, substitution is definately a much better option than dilution. Just get a 2 oz travel shampoo container and keep it under your nutsack. The only thing you'll need to worry about is the temperature of the urine, from what i understand it needs to be between 90-100 degrees (preferably between 96-100 degrees). Get a thermometer and practice before you do the actual thing. If you can't keep the temp up with your body heat alone, then buy a heat pad of some kind ( maybe one of those thermacare heat wraps).

I'm in the same boat as you, i have to take a pre screening drug test tomorrow afternoon. I've been practicing today and everything seems to be going good, my temp has been staying right around 97.3 even after 3 hours, and thats without using a heat pad. Good luck, hope you pass your test!