First you rig up a portapotty or a regular toilet so when someone sits on it to take a dump, water shoots up at high speeds. If you do it in a toilet, they'll get more wet and it would be easier to get away with. But if you do it with a portapotty, you'll have all the pee and poop go up and get all over the victim. I'm not sure what would you use to shoot up all the "liquid" so ask a scientist or something on that one. Have a pressure sensor or heat sensor on the toilet. And to make sure someone sits on the toilet, maybe slip some EXLAX or something. Obviously don't make it too dangerous, but enough to scare the shit out of them and on them.
Have a camera inside the stall, and outside. (Yes outside, it should be that epic.)
Yes I'm from South Dakota.