While not complete, I'm posting the CAD of this in case anybody would like to take a look. We'll likely develop this in to our 2015 drivetrain, but for now it remains a partially developed concept.
We have built 3 robots using the Vex 2-CIM ball shifters and overall we've been happy with them. We have used a method of attaching them to the rails very similar to what is in this CAD model and it has been rock solid. It requires some modifications to the way the gearboxes are assembled, in particular replacing a few of the hex standoffs and bolts with longer versions and adding a few nuts. This season we may migrate to a 3-CIM setup if we think it is worthwhile.
Caveats: the shafts are not included, the bumper attachment is still a work in progress, lots of fasteners are missing, and in general it lacks finish work, but I think it is a pretty decent start.
CAD here:
2811 Octagonal WCD.zip