We use the encoder attachment on the 3 cim ball shifters by vex. (
http://www.vexrobotics.com/3cimballshifter.html) It makes it super easy to attach and use encoders though the gearboxs in general are quite heavy and expensive. In addition we have had some trouble with the retaining clip for the shifter shearing off. We use the entire line of WCD for our robot and have found that this is the easiest wasy to mount encoders.