I agree that machining the curve would be the cleaner, more professional way to do this, but if you need a simple solution, you could mount two lengths of round stock on the edges of the base that are parallel to the centerline of the tube. You would have to be sure to mount these equidistant from centerline of the focuser tube to prevent canting off center. With some figuring, the base will just touch at the apex of the circle while the round stock provides the 'feet' for mounting. A bit of foam rubber could be used to 'seal' light and dust from coming in from the bottom of the base.