I've found a source for contra-rotating gearboxes for those of us that are mechanically challenged. The units are very simple using 4 non-metallic bevel gears in aluminum square tubes. John Regalbuto, the builder, also has other configurations and I'm sure, given the specs, he can build it. Everything he uses is "off the shelf" and replacing any parts will be very easy. The unit I ordered is small ( 1.25" square tube x .688" thick ) and swings a 6" prop for airplanes. I'm looking for a source for small stainless steel tubes but I'm not that concerned, for now, because to change the shaft just loosen a set screw and slide a new shaft in (K/S brass tubing).
For more info contact John Regalbuto at email@example.com