I used Solidworks to design some frames to “sandwich” the rear windows into position.
The old windows were bolted directly to the window frames (yellow arrow) and I’m sure that’s why many of the bolt holes in those windows had developed cracks. I didn’t want to make new frames, but a problem developed when the new fabric was shrunk into place. The bottoms of the old window frames developed some wavy bends (red arrow) from the tension of the fabric pulling against them. The easiest way to address this was to make new frames that would sandwich the window and the old frame together. Therefore each window assembly now has a frame on the outside and inside (à la sandwich).
They turned out nice.