Here is where I am- just as you describe. A small printed rotor against a flat seal plate with a hole. The curve is “area ruled linear” for the hole as it turns through 90 degrees. The plate was going to have a oring for seal and some seat pressure but I think I will try some foam tape as all orings I have are on the stiff side. The seal plate and rotor will be in a printed “box” that attaches to the top of the servo using the original mounts.