I have an evaporator from Thor (quebec mfg). it's a 30" x 8' raised flue (7" flues). the part of the arch that is under the flue pan is just like you have made. however being an 8' long arch, there are two ramps and drop offs. that rig boils awesome (was getting a consistent 60 gph and got up to 67 pushing it... stock, bare bones rig). however, it also eats wood! i dont' have anything to compare it to, but it was not easy on my wood pile this year. i'm sure if it was air tight, and had aof it'd be a lot more efficient!
Awfully thankful for an understanding wife!
“The only place success comes before work is in the dictionary.”
- Vincent “Vince” Lombardi
Good luck to all!