Practice and patience. Originally, I ordered the 1.5" x 1/2" size as I had seen this used as well but I got a little too eager and didn't factor in that my drum has a rather substantial lip and that might potentially be a pain to deal with when inserting the screws so that 1/2" of the aluminum rose above the drum. It was at that point that I realized that the extra 1/2" was crucial and and I moved on to the 2" size and took my time. Lesson learned. I even drilled the holes in the aluminum first, so I have a guide for the drum.
I deliberately made it a tad too big to start, then shaved it down slowly; first by taking roughly 1/8" off of each end with the jigsaw, then switching to a file for fine tuning. Now it looks nice...with just the slightly loose lid.
I could probably hold off, bolt it down, then seal it with Permatex and see how it holds up when I go to season it but I was curious if anyone has tried something other than fiberglass, as that stuff is a pain to work with and I'd rather not have the safety risk.