Thanks again "Cruiser" for all of your added info. I totally agree, how much to cut out is pretty much a guess. With the sides being more contoured, they will have to retain enough of the fiberglass structure to keep the shape as it curves up and across. Otherwise the fabric would pull straight across the void rather than curve. But as you say, strength wise those sides are the strongest portion so they don't seem to need a lot of structure. The front and rear ends are limited by the attachment hearware mounting surfaces. But you managed to cut around them very well. So a good balance between reducing weight and retaining shape and strength.
I'm impressed at how well the fabric retains the overall shape without showing obvious depressions where the cutouts are. That was my hesitation with using a fabric cover. I was thinking of maybe a thin aluminum skin. But I like the fabric much better.
Just for reference. Here is an image of how one of our vendors cut the fiberglass away to use it as a framework for a plexi covered top:
But in this case it would not need a lot of structure due to the firm cover that will be added over it.
In the past we have thrown around ideas for something like your light weight top. I've always thought it would be nice to have a collapsible framework with cloth cover, like the old Porsche Targa tops had. That way it takes up very little room when removed. You could even store it behind the passenger seat rather than going in/out of the trunk, freeing up all that room and being more convenient.
One possible simple design might be something like those camping tents, with fiberglass poles to stretch the cover tight. The front and rear edges of the original top would be retained to connect it to the car. But the space inbetween could be some removable poles. The fabric is attached to the front/rear end sections so the whole thing could be collapsed by taking the poles out. Sort of looks like this (the red lines are the poles):
The entire space between the ends of the poles is removed and replaced with fabric. You might be able to carry the poles further rearward, with more area cut out. The major drawback is the lack of side frames and side-window seals. So definitely a summer top. Maybe a good candidate for the mesh fabric I referenced earlier.