Back in the 80's there was a set of chisels for cutting notches in ribs and formers. I think Balsa USA used to sell them. You used them kind of like that small chisel finish carpenters use to square up corners on routed inlays for door hinges. You put it over the corner and tap it with a hammer and it squares it off. They had several sizes up 1/4" if I remember correctly. I looked around but can't seem to find them anywhere. I wish I had bought a set then.