Should you want to build your own, here is Jim Z's paper on designing a 4 speed transmission. The 4 speed was a solution when there were not as many powerful motors.
http://www.killerbees33.com/index.ph...82&Itemi d=50
One of the good things about this paper is it talks about making a torque/power gear map. If you are goind to design a transmission this is something to keep in mind.
That being said we did Dewalt's 2 year (2006, 2007) and then used AndyMark this year. We rebuilt the end plates (steel) into aluminum to save some weight.