What is your preferred way of mounting them? If you don't mind my asking...
It is similar to the Robo Challenge mounts in the sense that I support the motor at the gearbox end. Basically, I tend to use a bulkhead design in my robots so I cut the back-to-back-D shape of the front of the gearbox out of whatever material I'm using for my bulkheads (usually some form of plastic). Usually the depth of the gearbox at that part is 20mm so my bulkhead material either has to be 20mm thick, or if it is thinner, I have to cut the same shape out of a thinner piece of material and use it as a spacer. I then bolt the whole motor in place using the mounting plate supplied with the drill (this is a round black plate, maybe around the 50mm diameter mark with two holes in it that you will see when you remove the chuck from the drill. It holds on the torque ring and you need to unscrew it in order to remove the torque assembly)

I should probably get some decent pictures up of what I mean because I feel I don't quite explain it as clearly as it could be. Essentially though, it is a pretty quick process and I've never had a problem with drill motors mounted in this fashion.