It really depends on how big you are talking. If you are talking big, say 8 or more feet across, you could make a frame of wood to fit your radius, put on some lathe mesh and make it out of plaster. This would probably require the services of a plasterer to get the surface smooth.

I made a small one about 4' across using plywood, 2x4s and masonite. Cut some 3/4" plywood so you have 2 pieces that are the required radius with a 90degree corner opposite the radius. Use the 2x4s to make a frame so the two plywood pieces are the desired width apart. Run the bottom 2x4s about 3 feet out from the bottom end of the radius. Screw the masonite to the facing edge of the plywood and use a piece(s) of masonite that is long enough to run from just above the top of the radius down on top of the protruding 2x4s. If you are using more than one piece of masonite, have a 2x4 behind the seam as part of the frame. butt the edges and screw them both to the 2x4 (countersink all the screw heads). Use a wood rasp or sand paper to match the edges perfectly and then fill any seams and screw heads with spackling compound and paint.

Hope this gives you some ideas.