Ellis Brown went to a lot of trouble to make a concave roller tool to form the profile of the under-rib for his .450 double! Whew!

A concave lower rib can be easily made by cutting a thin strip length-wise out of a length of suitable-diameter steel tubing. A gunsmith friend of mine Vic P_ did just that last week, on a sleeved DR which arrived for regulating with only the top-rib!