I can't remember where I got them but you can buy drop rags that act like a diaper. Check the auto parts stores. I really like them because you can get them out fast and they don't make a mess. I wouldn't take kitty littler because if you spill on the rocks its a mess to clean up. When I dumped upside down in one of the hot tubs last year at Moab I had those diaper rags and I was amazed by how much they absorbed. Probably around 1-2 quarts a piece. Not bad for a glorified rag. They're also easy to carry and pack.
i used to have a little dust pan and broom (bought in the camping section, for sweeping out tents, was $2 I think) that was in the bucket for cleaning up the oil soaked dirt/kitty litter, but I stepped on the dust pan
for me, the biggest thing for the bucket isn't so much the disposal, but the ease of throwing broken/greasy/nasty parts in it, and not having to worry about wrapping them up, or wedging them in the bed somewhere.
