Bought some MDF board, trimmed it to fit, and laid down some adhesive shelf paper before securely mounting the equipment...
...then test fitted it in the car. This is the rear of the car with the seats folded down. I can easily tweak the amp from here without crawling around in the trunk.
The stray wires were left over from the previous owner's attempt to add a sub to the factory system. I will be cleaning those wires up when I get ready to mount things permanently.