How about the Off-Road Commode, the portable toilet seat that hangs off the back of your truck from the trailer hitch receiver. It comes with the following warnings -
Not to be used when the vehicle is in motion.
Can get Slippery when wet.
Off-Road Commode, Model# ORC | Hitch Accessories | Northern Tool + Equipment
I wonder if it would be ok if it had a seat belt and you wore roller skates?