Unfortunately random drug tests are hard to fool. Normally, your employer selects you for a test and you have to go immediately (The official DOT rule is "immediately before, during, or after performing the duties specified under section yadda yadda").

My recommendation to you would be to keeop yourself hydrated anyways (it's good for you) and find a way to keep a whizinator or some other source of synthetic/clean urine avaiable. Make sure you hit the temperature right, that is one of the first things that is checked. Anything more than a few degrees higher or lower than 98 degrees and it's an instant red-flag.