I suffered with that same malady in my ‘83 everyday driver Avanti….
Fed up with it and having the ability to do basic bodywork, I purchased a sheet of 1/8” virgin fiberglass and filled the opening in.
I left the sliding moonroof intact… leaving it in it’s dropped position, but not rolled back.
This gave good support to the new fiberglass roof panel, which I carefully cut & trimmed to fit into the factory roof opening.
I then ground the fiberglass down about 4” around the perimeter of the factory opening, tapering it.
I did the same around the edge of the new fiberglass panel, and put weights on it to hold it in position. (it’s slightly curved)
I then used fiberglass resin and cloth joining the new panel with the factory roof opening….. I then ground that down after it cured… then finished with a skim of body filler, primer and paint.
I made this modification on my Avanti about 15 years ago…l have no regrets…. and a dry head!!…….Ed
PS…. On the inside I made up a new headliner out of a composite sheet ….from the roll bar cover to the windshield header… and painted it a close match to the interior color (tan)