Some 50 yrs ago or so;, I camped at great Sand Dunes on Lake Michigan near a troupe of ?? boy scouts, and watched all ?? of them wash their hands before meals from a 2 quart can with a nail hole in it (plugged with a match stick when water not needed). In a modern age, a nalgene bottle with a microvalve (ala drip irrigation design) would suffice filled with warm water and suspended at appropriate height. It worked for me working in the deserts of Baja California 30 years ago (even in winter) so why not today. 1-2 liters is adequate to wash all parts and even hair if you are not wearing long locks. Wet, soap, rinse.