My advice, coming from my first year of evaporating, is to insulate insulate insulate. No matter what type of burner you end up with, keep it sealed well. I made my evaporator from an old fuel oil tank, making the mistake of NOT using fire brick (or anything for that matter) on the inside of the tank. I also didn't use anything to help seal the small gaps where my pan sat on the burner. These two things combined resulted in several hours of burning for minimal results. The last burn of this year was about 7 hours to boil down only 25 gal. of sap

next year WILL NOT be the same for sure.
Just for kicks, here's my burner without the pan