Not for use while in motion...

[G-man]

Can you imagine what a bear would be thinking if he saw you actually use one of these:

Off-Road Commode

Bears will wish they had one! Every sportsman needs his own throne, and the Off-Road Commode fits the bill — with comfort and luxury to boot! Easily attaches to any 2in. receiver hitch and supports up to 500 lbs. The 1 5/16in. dia. steel tube seat is covered with soft, padded camo. A great gift for hunting, fishing and camping buddies! Not for use when vehicle is in motion. Can get slippery when wet.

HERE

[The_Muppet]

Also called the "Bumper Dumper" - I've seen one IRL, but didn't use it  .

[Oddball]

Seen the Top Gear bods use one on thier driving challenge to the North Pole/Artic Circle.

[cotejy]

Wow, wish I saw this post a few weeks earlier. I was in the bush forest for the last 10 days and could have used one of those. I think I'll take the idea and try to build something like this to put on my ATV.

[Baradium]

Seen those, although the ones I saw actually used a real toilet seat.    You hooked a trash bag up to it and everything was even prebagged for you if you had to pack it out.