Take two square pieces of steel big enough to lay your workpiece on and a hard dowel pin. Drill one hole through the center of both piece, tight enough for an interference fit. Ream one plate to make a sliding fit. Press the pin into the interference fit. Bolt the piece with the pin in it (pin up) to your table and indicate it to center the pin under the spindle, and note your coordinates. Bolt your workpiece to the other plate with countersunk screws from the bottom, with the part located with the arc the appropriate distance from the dowel pin center. Use spacers if you want to preserve this "fixture". In a place that does not interfere, bolt a long handle (e.g. several feet, long enough so that it can't make a complete revolution without hitting the mill frame) to the top plate. Apply grease between the plates and assemble. Fix a small diameter (for safety) endmill in the spindle. MAKE SURE YOUR ENDMILL HELIX/SPINDLE DIRECTION PUSHES THE PART DOWN, NOT UP. Move the table so that the desired radius (include half the endmill diameter in your calc) is achieved. Standing outside the radius of the end of the handle, turn the machine on. Remaining clear of the handle, you should be able to slowly swivel the part to cut your arc.
Or, for a a couple hundred bucks buy a used rotab on ebay.
Note the small diameter end mill (it will take longer with a less-nice finish, but the small diameter imparts a lower torque on the part from the machine spindle), long handle (high torque on part by operator, operator standing outside the radius of handle) and helix direction (machine does not spontaneously disassemble your fixture). You could drill and ream extra holes around the circumference and drop pins in them as you progress to limit uncontrolled travel of your system, or you could use a sleeve with a countersunk bolt instead of a dowel to ensure the top plate is held down.
J