If the darker brown parts are tubular and you can get though the ends of them you could cut some wood and pass it inside of the tube with a snug fit (with occasional screw from the outside to keep the wood from sliding out). (Could even be something to screw the angle iron into to pull it straight.)
If yo worry about the wood warping after you have the size right coat paint or otherwise coat the wood.


Note I have never seen that type of garage door up close.

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84 300D, 82 Volvo 244Gl Diesel