So, My car came with some 4.5" speakers install in the dash. they were mounted with some adapters made of MDF, which eventually crumbled, and left me with some pretty nice speakers falling out of my dashboard and onto my lap.
I got tired of that, and tinkered around with my 3D printer to produce some adapters.
These will let you install a 4.5" diameter speaker in the dash. There's one STL for the left and one for the right. You can also tweak it to whatever size you want.
I recommend printing in PETG or other high temperature filaments. If you use PLA, you'll probably want to anneal it before printing. If you do that, be sure to print the first few layers, anneal it, then look at the shrinkage and tweak the model.
https://www.thingiverse.com/thing:4252152
Please let me know if you you try it! I'll post some photos soon. I've already printed the left adapter.
For the right adapter, you have limited room, which is why I tweaked the settings to offset the center. The 'speakerrings.json' has the values used to make both STL files.
--buddy