re: How much water will cause a diluted urine sample?

Supposedly you wanna "mid-stream" it, and especially avoid the initial pee. If you have nothing to worry about, drink a little less before going in, so your pee is yellow.

For those who are super desperate:
Use a travel shampoo bottle to store clean urine you have obtained from someone (handle carefully). Place in warm water en route to testing facility. Get one of those laser thermometers that tells you exact temperature. Make sure the sample is about 110 degrees F, maybe a little higher, but not 115. Put the shampoo bottle in your underwear near your gouche (boxer briefs normally have a compartment you can slide it into). This will give you a 15-20 window to walk in talk to the receptionist, do what you need etc. before having to give the sample. Then just don't shite your pants.