Fortunately, I was lucky enough to find a donor piece...but I believe the corners could be fabbed if nesassary. BTW--if you do fab the corners, those "pockets" that are there, are NOT needed...all they did was provide the factory an easy place to spot weld the two pieces together...oh, and trap moisture causing them to rust!!
Also, make sure you remove the weatherstrip retainers while you are there. GM didn't paint under them, or seal around them so the moisture would creep underneath them and that is where the rust starts. Get the stainless ones from G-bodyparts, they work great if you use body panel adhesive to attach them. Using the adhesive also seals under them, preventing future moisture problems. Keep posting up with more ?'s and I'll help if I can. Also POST SOME PICS!!
---Keith