One alternative to a flush toilet is a composting toilet. The cheap-but-sanitary way to handle this in an enclosed space is 2 5-gal buckets, one with a toilet seat mounted on it and the other filled with something naturally absorbant/drying (sawdust, sand, fine shredded leaf litter, even kitty litter if thats all you have on hand (not sure how compostable that is though) or a combination). Each time you use it, just cover with a healthy scoop from the other bucket. Dump the bucket when full. If you're in a long-term situation you may want to check out the book 'Humanure' and see how you can use the waste as fertilizer or at least how to keep the dump site from attracting attention/smelling.