Re: Best way to cover my 6x9 speaker holes
If it was me (and I'm sure there are dozens of other equally valid answers, and probably some which are better) I'd get some 1/8 fiberglass sheet (McMaster sells it, there are other sources as well). I'd cut out a rectangle from the sheet, radius the corners, and spend a little time sanding it so it will look somewhat decent when painted. Then I'd install it with some round head stainless screws, with a bead of door/window polyurethane caulk to seal it (this isn't going to be submersed so the exterior polyurethane caulk will be more than adequate).