So far mine is really basic. I am still running the first draft. I really like the simplicity of it and it gives a surprising amount of extra room in an already cavernous wagon.
It is plywood and MDF screwed together, creating a flat surface all the way to the front seats for storage and winter sleeping. In the rear cargo area I have two slots to slide in a couple rubbermaid?/Target underbed storage boxes. I have found this extremely flexible in that I can swap out different boxes for different purposes.
The other thing the simple design affords is more nooks and crannies for crap. I find myself using the space across the width of where the back seat would be for all kinds of things- chairs, CO2 bottle, tripod, boots...
Version 2 is going to be taller (haven't decided how much) and have more MDF near the front for the subwoofer/amp box. Have to come up with something for the rear.. flaps or something, as I have wagongear.com rear fender storage panels and need room for them to open.
Ultimately I am going to be using an HJ61 split 50/50 rear seat and wil have to work out a modular way to replace the seat with the panel when I don't use the seat. Not sure how it will all work out, but it is fun to build and play with designs, and check out what other people are doing.
Wish I had better pics, but you can see the basics here-
The grey box is a generic 10" sub box pointed down which I'm replacing with an all-MDF enclosure