I find that one of THE BEST containers to do this in are those contact cleaning solution containers. You know, those "Opti-Free Replenish" containers. Hardly any noise, you just kinda lift the cap off, and squeezing it kinda simlulates a pissing motion. You can go in with those big bottles, and not even take a piss, just fill the cup out and squeeze the rest into the toilet like you actually pissed. But for those big suckers, you'd want some baggy pants, and you'd want to strap it to the side of your theigh.