Here's what I've done in the last 24 hours...
Masked off the corner area with 2-inch blue painter's tape and plastic.
Laid down several layers of fiberglass matt and cloth (over initial layer of fiberglass resin).
When (mostly) dry, I pulled out the fiberglass (with tape and some plastic still attached) to continue to dry out of the car (it's sticky AND stinky!)
Here's the fiberglass with most of the tape removed but I hadn't yet started to trim down the excess and/or sand. This is looking at the backside, which will be hidden when installed into the car.
Next steps include adding more layers of fiberglass to the interior of the box and possibly some MDF pieces for support. Issues still to resolve: 1) am I going to use the base portion I previously made to attach to this box for added interior volume, 2) should an amp rack be added to conceal all the wiring or merely just a solid MDF piece to attach the amp to but still have partially exposed wires (would concealing the amp lead to overheat issues?) and 3) what to finish the box with?