One thing we used to do when climbing was take a "poop tube". All it is is a PVC tube, maybe 2" diameter, with threaded ends to screw caps on. To use, you do your "doody" in a bag (we used paper lunch bags, but plastic would do fine-those for pet waste would work well, actually), put a handful of cat litter into it, seal the bag, then put it in the tube. Seal the tube, and viola! Granted, you will have to empty the tube eventually, but it certainly works short term.