Just so we are speaking the same language, when you say "spring" & "sag" I'm picturing what mine did; it opened up a little to a new larger radius. I used a continuous piano hinge across the whole top opening. Welded it up tight, put a 2x4 longer than the door is wide at about the midpoint of the opening and forced the bottom edge to pinch in with straps, chains and load binders a little at a time. It worked but it was a pain in the arse, not an optimal fix. One of the other recent build threads had a pretty good solution, I'll try and find it. As I remember it was essentially a pipe clamp to pull it back into round. Maybe someone else can add a helping hand here.
Here is the link I was thinking about, look about 1/2 way down in the pics & narrative.
Maybe Pappy can help out. Looks like a much better solution than mine.