Google "stove top still" and look at what pops up. I did this earlier today. One link was for water purification and they were quite proud of "1.5 quarts in 2.5 hours". So, in about 8 hours of steady fire tending, you'd get a gallon.
Lots of these little stills use water jacket cooled coils. A motorcycle radiator might do the trick for portability, as a condenser.
Now you got me interested. I paddle a Hurricane Tracer which is low volume in the holds, so it needs to be a really compact setup.