Depending in how big, bad the corner is, you can use nails to make a mesh/matrix for the bondo adhere to. Tap them into good wood. Lay the cab on its back, use a piece plexi or the sort to make forms. I use tape to hold the plexi form into place. Drip the bondo into the form. When its dry, pop the plexi off. The bondo dont stick to it. I use a belt sander to rough form then use a glaze coat for final.