I use a 5-gallon plastic bucket! Not directly over the heat, of course, but filled with smaller pots of boiling water with water from the hose added to bring it down to the right temperature. The water doesn't need to stay at 150 degrees all afternoon while you're processing, just when you're ready to scald & pluck. You can keep smaller pots filled with water to boil on your kitchen stove or over a campfire or gas grill.
The next time I process I'm going to try using an electric hot-plate set out near my processing station with a long extension cord. I'll set some bricks around it and place an iron grill over the bricks, so the heavy pot isn't setting right on top of the hot-plate. Then I don't have to fuss with a fire and having smoke blow in my eyes while I'm trying to work.
I usually process alone, and do the birds in small batches, 4-6 at a time. I do them in pairs, so I only have to make scalding water 2-3 times. I have a teen son who will carry the water out from the kitchen for me.