OK, here's what I've come up with so far. It's kind of shade-tree, but I'm not real good working with wood. There aren't a lot of nut holes that will work without fabricating brackets, so I used these 4 holes. I also had work around the power back window cables so I wouldn't damage them. It doesn't have to be real strong. I'm not going to haul boulders or heavy toolboxes back there. I'm not sure if I want to permanently mount a floor for it. I like to keep a small ice chest and trash bin just behind the front seats. The top board is 21" x 55" and the floor if I ever use it is 24" x 55". Eventually I'll find some tan carpet and some insulation but it's good enough for now.
A 2x4 mounted to the two holes up top, and a 1x4 bolted to the bottom two holes.
Then I screwed the board to the boards.