How to make the back fit more tightly

I have repaired and upgraded many iPods, mostly 5.5G, but a few 4,5,6, and 7. I have been buying aftermarket backs and find (as one might expect) that the specifications are a little loose. I am currently working on a 7G, but I expect that my question is equally applicable to 4-7G since the backs attach basically the same way. When I reassemble the case, it is not coming together tightly, so the seam is not tight. Before I “trial and error” it, I thought I would ask the question here to see if I can start with the best method.

I tried bending the flanges out slightly. No improvement. Is there a difference between the backs of a thick 5G and thick 7G? Otherwise I think it may be just a sloppily made back. Thanks for any assistance.