Not sure if this helps but one old school trick for these situations when any extra space is a blessing is to actually build the box onto the back of your cab. The rippled metal behind the seats is the back of your sub box. Basically you place a panel up against the metal then use a compass to trace the ripples, then cut it out with a jigsaw. Use those for your top and bottom. Basically just build a box with no back with these rippled panels, then screw the whole thing on. Put DynaMat on the metal, possibly screw a brace or two on, seal it up with silicone. I've built one of these in a Ford Ranger before.