This is a simple, efficient 15mm thick bracket I made to mount my 6.5" speakers into the 5" hole on my car door. Works on any speaker size. Super simple design but works like a charm. There is a hole so you can rivet the bracket to the door panel and then use a self tapping screw to mount the speaker to the bracket.

I tried to use thicker infill (40%) and walls (4 lines thick) so the self tapping head stayed sturdy while 4wd. The settings I used were more than enough and you could easily use less. Super quick print time too.