Behind the seat rear wall ideas?

[JAG66]

In all my square bodies, I've had to re-do behind the rear seat because the cardboard tray was always tired looking or missing leaving an unfinished look. I still have the factory speaker mounts and grills, but need to buy new 4 X 10 speakers.
Do I carpet everything, build a durable tray, or just replace the cardboard tray?
Please provide some ideas and pictures on how to finish off that space behind the seats to give it a nice finished look.