Swamp all the flats except the bottom flat.
When filing the 2 oblique (sp?) flats on either side of the bottom flat, you will form a narrowed flat on the bottom. It will be the shape of the 'swamp',,,going from wide to narrow and then back to wide again.
But it will be a flat surface.
To this flat surface attach your half rib for the ramrod. The easiest way to fashion one that follows that swamped sided form is to make one from wood.
The attachment surfaces of both the rib and bbl are flat, so that is easy to take care of.
You only have to shave the rib sides to match the swamp of the bbl to make it look right.
The bottom edge remains flat so the ramrod runs smoothly down the rib and through the guides.
Don't do too radical of a swamp on the bbl so the rib doesn't need to be shaved too thin side to side.
Pin the rib in place using underlugs dovetailed or soldered to the bottom flat just like normal underlugs.