I have self supported the canyon as well. I even own two groovers, one for shorter trips and another for longer ones. So from personal experience, I would have at least 40lbs of $hit in my boat if I was in there for twenty days and didn't starve myself. That equates to a 1000 cu inch groover.
Now, some people will empty their groovers at phantom and the other couple toilets located along the run. If you are really bold you can empty it in an unsuspecting rafter's groover(This could be a discussion in itself). All I'm saying, if your not going to be all weird and creepy with your poop, there is going to be two personal groovers worth at the end of a twenty day trip. At least for me.
Man, I need to go kayaking, talking about poop on the bzz.