What they did in the picture was use mdf for the whole box, which is why they have to go higher (that compartment area requires many angles to get enough air space with wood). I did a similar setup the first time I ever did one of those trucks a few years ago (used 2 8's)...I learned quick it's not as near efficient (or even as easy) as using fiberglass. What I do on them is make just the top plate out of mdf. It sits identically (like a template) on seat base. Then the entire underside is fiberglassed to form to all the curves and such of that compartment area. Not saying you couldn't do the whole thing out of fiberglass...this was just the easiest/cleanest/most "oem like" way I found. With smaller subs I would even flush mount an amp rack right into the middle which made for a really clean look. Matching carpet and you'd swear it were factory.