I never hd any evaporator with a bottom or ash pan in it. My Half pint sat on a concrete slab, no issue. My 2x6 Leader drop flue was on a wooden floor. I made a platform using 8x8 16 concrete blocks and I used a piece of 16 ga galv tin as a floor under the fire, no problem. I laid the blocks so the openings were out the sides, the top was solid. It worked very well. Because the floor was wood, I made the block platform over sized. I think that base was about 18-20" wide, my platform was 30" wide, in front I had an apron of concrete blocks that extended out front about 30", also capped with tin, but I think that was 24 ga, and around the edges on both I had bent the tin down at 90 degrees about 1" downward, thus to start each was over sized 2". That worked very well for 4 season.
Then I removed the wooden floor, filled the area with crushed stone and under the arch area I had footings down 4' and concrete blocks laid up for support. I filled the boxes formed by the blocks with crushed stone and then poured 4-5" of concrete floor.
My next evaporator was a 3x8, no bottom under the fire, but it sat on the concrete floor, never any issues.
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.