I found a textured vinyl at Joanne's fabric store that is dang near perfect match to the door panels. Here's how I installed a 6 3/4" speaker in the rear doors with no mod and the ability to go back to stock:

1) remove door panel, unplug speaker, unscrew speaker from door panel, unscrew nuts that hold speaker grill on, put door panel back on.
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2) use stock grill as template, make new insert out of 1/4" Masonite, cut new speaker hole in insert, stretch vinyl around new insert.
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3) install speaker, wire to stock speaker wire, use 1 1/2" drywall screws to mount speaker to insert + insert to door panel.
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Solid as a rock, sounds great, no mods to door panel, factory speaker and grill and go back in.