Oil canning????

My issue has 0 to do with a car body but it is metal related. Working on my reverse flow smoker. It is based on a 60 gallon air tank compressor. It is built out of 1/8th inch mild steel. I cut the door out of the cylindrical portion of the tank. When I first cut it the door section fit like it should, it followed the curvature of the tank. After welding hinges on the top and a handle towards the bottom....well....it fits like crap. The bottom edge, near where the handle was welded, has a large bow in it. I put some 3 inch pacers under the lower corners and cranked on it with some clamps. Between that and some heat and hammer work the lower edge was "close enough", considering a gasket/clamps will be used. Had a problem with the hinges on the top. Cut them off, welded on another set. The bow returned. What do I gotta do to get it straight again??