The regular old solar still seemed to require an enormous hole, 3x3ft or 3x4ft. I always assume that you needed the big hole to expose more damp soil for distillation. But if you've got a smallish hole (maybe a ft in all dimensions) with mud in it, maybe that would work. Water flows to the lowest point, & if you condensed what was in the hole, maybe more would drain into it.

This is just theory, mind you! But so were solar stills.... <img src="images/graemlins/smirk.gif" alt="" />