Reply by Jorge G.

Cosmic’s solution is the only way, I have used the iron square trick to straighten beams.

I think the question you should ask yourself is why did it bow? Now you know your construction method was not appropriate. If I am seeing your diagram correctly, you screwed the front part to the frame, a 1.5” frame this long is not stiff enough to prevent the wood from moving, which is what happened to the front.

-- Just because you’ve always done it that way doesn’t mean it’s not incredibly stupid.