Perhaps review this thread. I followed this thread exactly and after getting everything properly tightened there was no leak. Like you I think I needed 4 washers on the outside, but the inside of mine is exactly as in this thread. I also used the o-ring shown in the following thread on the outside of the bulkhead. I wouldn't worry about the curvature of the cooler. It's plastic and your other parts are metal. With proper tightening the plastic will give enough and the washer on the inside will seal. Note that the washer on the inside is the one included with the cooler. I'd pry that sealed washer off, clean up any sealant, and restore the factory washer and reassemble.