But about your original door panels, do you think the moldy mush of the particle board could be revived with the application of some epoxy brushed on and soaked in to stiffen things up? perhaps along with some mesh drywall tape or other suitable reinforcement?

That might allow you to continue to use the panels as a means to hold the speakers, with those clip-nuts and screws. That method is preferable (IMO) to going through the panel and into the metal door frame, preexisting holes or not.

In the final iteration of my car stereo days I managed to fit quite a nice 4" midrange speaker and aftermarket grille into the door by making some adapters out of wood. The first piece was a spacer, round but with the top and bottom cut off, to fit the recess of the door panel. Then the second layer could be made the full round of the grill, thus providing a surface to mount the speaker and grill. It also bumped the speaker's large magnet out far enough so that it wouldn't interfere with the window glass.

Best of luck,

-Will
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XC60 / Odyssey