I'd use Little Tappers ideas but modify. You don't need to weld the steel sides, they can be welded or bolted. For the metal sides I'd suggest either 20 ga SS or 18 ga galvanized, either will work fine, the SS will look good much longer. Fill under the flues with vermiculite, available at large green house suppliers fine or course grade are good. Don't buy it from a big box store or you will pay several time more. In the firebox use half brickes purchased from a masonry supply (lower cost). See if you can get wheels that will retract or remove for in season.
Split your wood wrist size and have it real dry.
For the vermiculite, on my 3x8 raised flue evaporator I calculated how many of Lowes biggest bags I would need to insulate a 3x6 raised flue sap pan area. It came to over $700 back in 2008, I bought 3 hugs bags of it at a greenhouse supply at $23 a bag. When finished I had about 1/3 of a bag left over.
If you can come up with a high pressure blower such as on an old oil burner and blow the air in on top of the fire aimed down about 15 degrees it will greatly improve combustion efficiency. Look up a stickey post titled combustion efficiency in evaporators, it will help lot. More heat to the pan, less heat lost up the stack.
Dave Klish, I recently bought 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.