seems like this would be a great spot to put a trap door so you can access the inside for storage?Make them out of pegboard so it's easy to organize your gear back there.
I had some made of 1/4" plywood for a while. The bottom was carpet and the top was just finished wood (oak-faced ply). Didn't really like it, but it could have been saved with a couple yards of vinyl from Jo-Ann fabrics.
Then I built them out of 3/4" MDF as speaker cabinets for some low profile subs. Works OK, but not nearly as good as my real sub box did. It is quite good for the 6x9s that are in it. The whole thing is covered in headliner material. Had a topper seal leak a while back, so now some parts on the inside are swollen. Would like to do it over again someday and I'm not sure what approach to take. To make the sub box thing ideal, I think the side panels would have to come out a couple inches and lose that interior space.