Another way I've done, is to use an old stone wheeled bench grinder, and grind holding the blade vertically. In other words, the tip of the blade would be pointing at the floor, so you're using the half inch (or whatever) width of the grinding wheel, rather than the 6" radius. You can just round the edges off the wheel to the desired radius you want in the bottom of your fuller.
I've also heard of guys who use this priciple by replacing the blade of a circular table saw with a grinding wheel. Then you can take controlled "bites" with each pass by adjusting the height of the wheel above the table surface, and use the "fence" as a guide/jig to keep things straight.