I used some needle files to clean out the flash in the light recesses, and spent some time with my dremel and a chomping bit to get the ribbed stuff out. It took some doing, and precision was key. You can see towards the bottom of the new cutout where the stud for the screw mount is. I'm going to leave that in place for now.
Here's the piece of styrene that I'm going to use. I marked it up with a pencil, to give me an idea where to cut. I used an x-acto blade to get the general shape, and then used files and sandpaper to get it to fit perfectly.
Here's the final result. This is after about an hour of work, just on that little piece of styrene. Though it fits perfectly, I'm going to end up doing some more work on it later, as the paint will add some mass to the cut out.
This is a test fitting of the back door. I'm probably going to end up gluing this in place, and then puttying up the seam to hide it. There's no reason to need access to the lower panel once all the electronics are in place. I sanded the edges of this white styrene back door so it would lie flush. The laser cutting used to get it to shape left an uneven edge.