If you want even cheaper and lighter:
Lay out your top board (1/2" Plywood, Baltic Birch if you wanna be fancy, varnish it if you want it stainproof)
Now get Milk crates and lay them out to distribute weight.
Make a 3/4"x3/4" Cleat screwed to underside of top board to fit inside milk crate.
Put your gear in milk crates then offload when you arrive to destination and put top board back on.
Only bad thing is height can not be adjusted as milk crate is fixed height.
I really like your idea. I got a milk crate somewhere in my backyard and I'll use it to test it out. I'll probably end up buying 3 more, so I can get my 54" wide air matters inside and even.
Sometime this weekend I will play with it and find out how long of a plywood I need for this setup.
Now, I am trying to figure out what would be the easiest way to get all the side and rear windows blacked out.