Here's a nifty little shop made tool for squaring the flanges of ribs. It's cut with a 10 degree undercut to allow for spring back. I can't claim the design as I saw similar ones on several other RV builder's websites. Some used 14 degrees of under cut which was too much when I tested it.
I used an old hinge which is sorta' loose which seems best as it allows for a little flex when needed. I also found I needed to always have some blue painters tape on the parts that touch the rib to prevent scuffing.