I’ve had good luck with cheap panelling, with the smooth back out. Jerry is right, though, 6" radius is very small. I’m trying to picture where a 6" radius would be used – if it’s an inside corner, I’d be inclined to not fool with a curve that small, just make an angle joint. Won’t cost much space, and a whole lot easier!
You may want to include a 3" ledge with the fascia. Something to hold on to, put a beverage or throttle, or aside hand tool. Just needs a little more bracing and planning.
Try soaking the masonite in water for about 2 hours, then clamping it in place. When it dries, it should be ready to fasten permanently into place. Maybe you could use a piece of linoleum to go around that curve.
Styrene is another possibility. I used it for corners in my train room, and I’m sure it would take the tight curve. Can get sheet pretty cheap at plastics store in Yellow Pages.
A cheap source of styrene is “For Sale” signs at Wal-Mart. Get the biggest ones you can find. I use them to make my streets and bridge plates on my trestle. Small picture nails or a hot glue gun works great.