I built something that looked almost exactly the same as this for my 2 dr, once I have a few spare moments I will be building one for the new 4 dr. Its prety simple, plywood with some slide style door locks. The holes on the rear roll cage members are at the perfect height to lock into.
I sprayed the plywood with marine grade water sealer and then covered it with the vinyl. The expensive part was the vinyl, spent $100 on a roll but had enough to cover two of these as I made one for a friend as well. The other materials were all less than $50, including the plywood. You could cover with gray carpet to match the interior for much cheaper.
The back piece was done to prevent someone from just folding down the rear seat and getting under the lid.
Only changes I will likely make are to add some rails/channel to the tops to attach webbing straps if I want to put anything on top.
Don't mean to hijack a vendor's thread but hadn't seen any other responses from them and this is a fairly easy piece to make with a couple of hours time.
btw I know the one shown is without a softtop, the cover I made for my friend's was for a soft top, just an extra few cuts in the back.