Whats the problem? Take 2 strips of 3/4" plywood about 2" wide with straight edges and clamp one piece on the inside and one on the outside right on the bend line. Make the wood as long as you can. Clamp it tight with a few strong clamps so it stays in place.
Now take a broad flat faced body hammer and tap that flange down nice and flat and square. May want to do the ends first and put a couple of verticle clamps on top and bottom on each end in addition to the face clamps to keep the wood in place so you get a very straight line. You don't have to beat it into submission. It will want to bend and flatten back in place.
I like to use these cheap clamps from harbor freight for this type of work but you can use what you got.
https://www.harborfreight.com/catal...eight,f,Sale+Rank,f&q=c+clamps+locking+pliers