I used some wire grates that I had gotten from a store that was getting rid of some old shelving. I put four of these across the area that is between the rear PUP frame member and the rear bumper. They are zip tied together but are held in place with padded metal loom clamps riveted to the underside of the frame and bumper. They were the perfect width and length when assembled.

This was for light-weight items that were either too messy to go inside, or for other purposes. I hadn't actually decided what to use this for when I did it.