I suppose you could try a waterproof bag and lighter material than copper but, having built a still or two (science projects), I can tell you that:
1) the more efficient your cooling and vapor collection, the less fuel you'll use. An old pressure cooker would work.
2) copper tubing can be bought at Home Depot, one box is enough....aluminum tubing can't, and plastic doesn't conduct well. A chiller bath helps.
3) it needs to be heavy duty! You'll be messing with it all day. Use a wood stove maybe? Imagine how long it takes to boil down a gallon of water on your home stove a you'll get my drift. You won't capture but half of that gallon if you are lucky.
4) Stills blow up. A waterproof bag needs to have a constant relief valve. A pan and lid have one built in (lid leaks).
I know you can boil water in a paper bag, but eventually that bag wears out, as would a dry bag or whatever you are thinking of.

As a yaker, I could likely fit a 1 gal. aluminum pot in my rear hatch with a lid and coil nested....but I'd sure get the system perfected in my back yard before depending on it.


If you pursue this idea, I'd like to know how it turns out. If you have a beach fire going all day....and we yakers that sometimes....it might as well be doing some work.