I made a rack with 3/4 cabinet grade plywood, some I had left over from a remodel. I cut pieces out that fit down into the spaces in the rack. I glued these to the bottom of the piece I used to cover the rack. I then bored 4 holes through the flat parts of the rack and bolted the plywood to the rack with carriage bolts, with big wingnuts on the bottom. I also added 2 inch aluminum angle iron on the edges of the platform three sides, with holes drilled in up side of the angle every 4 inches for tiedowns and bungee cords. I covered all of the plywood, top and bottom, before installing it with Rhino liner. It's held up well for almost 2 1/2 years now.
I also added two 5/4X6 boards to the back of the cab with u-bolts around the cab frame to make a front for the deck
Made this .090 aluminum dry box for the Pioneer 500. Box is 12" deep and the top rack is 4" high. Has gas shocks to hold up lid and lockable latches. Mounts directly to the stock rack. You can see additional photos on my website alumilitearmor.com or feel free to give me a call @ 435-990-1012 if you have any questions or interested on having me build you one.