Someday when I get time I'm going to mold a fiberglass storage compartment insert for the factory hardtop, something like this:
One consideration for that (and for any use of the blank retrofit kit, which is why I bring it up now) is the depth available between the hardtop and the sport bar. This photo shows a tracing I made in a 2 door - I measured the depth possible at the top and bottom - 2" at the top and 4.5" at the bottom.
The compartment could be any depth desired in front of the sport bar, but at the sport bar it would have to be no more than 2" deep at the top and 4.5" deep at the bottom. It could be that depth across the entire width as in the concept drawing at the top of this post, or it could be deeper in front of the bar and that depth at the bar. I haven't decided yet how I'll make the fiberglass cargo compartment. One consideration - if the depth is constant across the width I can make one mold and the compartments from that mold will fit on either side of the Jeep. If I make it deeper in front of the bar I'll have to make left and right molds.
Some cargo compartment concepts: