Glad you are looking to doing this as well, First thing you need to do is gut the speaker so your just left with the ring like I did.
Im still waiting to get the final measurement you are looking for. I cant seem to locate a spare door panel to test with.
I could make it on my on door panel but then I would have to have my door apart for a day or two while the epoxy dried
etc and with a kid and working 5 days a week its hard to do.
Need to acquire a spare door panel for testing only, Get the exact measurement and after that I will put up a 100%
DIY Step by Step thread that can be made into a sticky hopefully for everyone.
This thread was just to gather ideas and interest.
13 F30 328i Sport 6MT
92 Sentra SE-R SR20DET DET Build Thread
08 E90 328i Sport [sold]
89 E30 325is [sold & missed]