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I guess I didn't explain as well as I should have. The 1/4" is the top of the brick lined fire box. this is the part that warps so much. The actual evaporator pan is made out of 16ga stainless. It does not warp. the plate warps enough that it puts a strong pitch to the pan and therefore creates a pitch in the pan with most of the boiling sap at one end.

A little advice if you would. I don't know much about steel and built a wood fired maple sap evaporator with fire brick lined block sides and rear. The top and front are 1/4" plate. The outside dimensions are 56"L x 48"W. It served my purpose but I was surprised at how much it warped as it got hot. I have a 23"x37" hole in the top that the evaporator pan sits over and the flame contacts the bottom of the pan. In the 48" width at first it probably warped 2" vertically and had a diagonal warp of about 6". I bolted 1-1/2" x 1/8" angle on both sides and the rear and one piece of angle across the width of the opening in front of the 6" flue pipe opening. It calmed the warping down some but I am surprised at the strength of the warp, it even bends the angle iron. The front has a barrel stove door and doesn't warp. What weight plate steel would I have to use to keep it from warping? Can a person have a piece of cast iron made? Would it be affordable? Any other ideas would be appreciated.