True, Kognition used to make them. But apparently they stopped production. either way they're delete wasn't even flush with the rest of the roof, it stuck out a little bit. And for the money you spent on it it didn't seem that feasible.
Your best bet would be to make a template out of some cardboard and either construct one out of fiberglass or some other material/metal. Bend some brackets and rivet them into the interior of the car. Then to make sure its waterproof you can line the interior portion with some clear caulk and lightly apply it on the outside portion, that way (aesthetically) it looks clean from the outside.
You'll save a good amount of weight high above the center of gravity.