My emergency back up are baby wipes. Not as good as a nice shower of course but 3-4 wipes (15 for a $1 or so) can get me reasonably clean except the hair. Fairly light, cheap, can find in most places that have a "travel" section that carry the miniature shampoos, deodorants, etc. If you put the bag in a zip lock bag, they will last a good month before drying out. Also, good if you run out of toilet paper.
Tailwinds, John