I've been doing a little research... unless you've got free access to a really nice CNC machine the cost would be pretty high. Unless you're willing to spend $70 or $80 for a 2 speed tranny then I don't quite think it would work. I think what we need to concentrate on would be finding a cheaper alternative. I'm not syaing it's not possible at all, it's just finding a happy medium between cost, quality, and weight/size. I'm wokring with a couple of ideas, but nothing has really stuck out in my mind yet...
Check out this article though...it's pretty good. I find the drawings a little confusing, but if you think about it it all makes sense.
RCTek Article
Jeremy