Whereas I can't say for certain, I feel like unless you hit the gym pretty damn hard you will still be dirty.

Pre-employment testing isn't monitored so substitution is a breeze. Just buy some synthetic at the local head shop or get some piss from a buddy who doesn't use drugs and put it in a condom or something. A big thing is temperature, you'll need a way to heat it. Fortunately hand warmers burn pretty damn hot, and urine has a pretty high specific heat (aka you wont make it too hot). Do some research to find out how to make the substitution urine 90-100 degrees farenheit (anything else is deemed invalid) and just fill the cup with that.