In thinking about it more, if your rib is only concave, "sunken", but is flat from end to end, the add on ribs will work. If, however, it is truly swamped from end to end you would have to have one custom built with "belly" on the underside.

It would still be way less $$$ than changing the gun's rib. Why not make one out of balsa, paint it black, and try it. 3M two-sided tape will hold it until the cows come home, and is removeable.

SRH

May God bless America and those who defend her.