1st and 3rd pic shows with the top off and how i have the amps and wires and stuff under there, 2nd is what it looks like with the top on. The brown thing you see in the 2nd pic is my bigass sub box that has yet to be carpeted.
Some light plywood(I dont remember what kind) and carpet from home depot and a few L brackets. I weighed it all before i put it in and it was about 6lbs vs the 40 or so the seat and stuff weighs.
it's pretty sturdy... i've had a few times i've had to have friends ride back there and my dog rides there all the time. As you can see it works good for hiding the amps too.