I sealed my back section off. I used plexiglass but I don't' think that would pass race car tech inspection.
The hardest part in sealing off the back section of the car is the weird shaped gap where the mechanisms for the convertible top normally reside. Its the void right next to the seatbelts.
Not too proud of it, but I used expanding foam to fill this void. I put painters tape on the underside of hard top. Installed the hard top completely. I cut some cardboard backings for the foam to fit against. Then sprayed the foam. The tape keeps the foam from sticking to the hard top, but allows it to press firmly and make a good seal.
To seal under the factory roll bar I just welded in a piece of sheet I had leftover from a trashed frunk lid I had laying around. Easy peasy. The aftermarket cage you have should be just as simple.
Then I cut some plexi to fit from that piece of sheet to the bottom of the hard top. I topped that plexi off with some D channel weather stripping.
I did make my panel or box out of angle iron, so rigidity isn't even a thing for me... if anything its more rigid than stock.
