Use a big crescent wrench to staighten out the bottom lip of the frame to make it square with the outside of the frame. If the two frame layers are seperated a bit clamp them together with vise grips then bend with the crescent wrench. If I'm straightening a section of the frame where it is only a single layer I use a peice of 1/8" x 1 1/2" x 4" long flat bar as a backing piece for bending the frame lip. If you are bending the lip up set the flat bar on the top edge of the lip and bend. If bending down put the flat bar on the bottom side. The flat bar keeps the end of the crescent wrench from putting a dent in the lip of the frame. If you beat on it with a hammer it just has a tendancy to sping back