Re: Distilling water
I figure anything impermeable would do instead of the plastic. Eg metal (from armour?) or even leather. I should have said that you collect this water overnight not during the day.
Re the rocks. I'm not sure the getting them hot in the day is important. I can't see what that would achieve, in fact I suspect it would work better if the rocks could be sheltered from the heat of the sun during the day so they are reasonably cool already in the evening. However if there is moisture in the ground under them then the stones would act similarly to the plastic/waxed cloth in that the moisture evaporating from the ground (still warm from the day) would condense on the stones as they would cool faster (being on the surface). Again probably works best if you soak the ground under the stones with the dirty water from the well. So maybe...