I just installed the quarter windows in my 28 special coupe recently. I used five or six pinch type clamps (Pony spring clamps) to hold the rubber as I really stretched the rubber very tight around the glass. I would place a clamp on the rubber, then stretch the rubber a few inches, then place another clamp to hold the stretched rubber. I repeated the process until I had the rubber stretched around the whole window. I then marked the rubber and cut it off. I then super glued the ends together with the rubber on the glass, but not all the way around the glass. Once the glue has dried for a while, I then placed the clamps on the rubber/glass as I did before all the way around again until the rubber was totally installed. I had no puckers in the corners. You really need to stretch the rubber a lot.
Hope this helps.
Rusty Nelson