Modifying window openings. Wrap glass around frame?

You can skin this cat a number of ways, but the quick way is to wet out the fabric, apply it and staple it around one edge, so you can pull it through the radius. You can staple the other side if you need to, but don't over think it. Pull the staple, clean it up, maybe apply some more reinforcement strips as shown above, etc. You can also make a mold to press down over the opening, forcing the fabric to conform, though you'll likely get some wrinkles. Bagging would be a good idea, particularly with well shaped and placed pieces, to prevent wrinkles, etc. I go the easiest route and do both sides, trimming what stands proud once green. Then I'll diagonally place strips or run the edge with pieces as necessary, knowing a grinder and putty will cure all ills.