Foam insulation will not be a good way to go. My suggestion is to find a used pan the size you want that if soldered it is leadfree solder. Then build an arch to fit the pan. This time of year is the best time to find a pan or pans, as it gets closer to next season it is often harder to find a decent pan. There are several threads on this forum by those who built an arch [the rest of the evaporator less the pan(s)].
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.