For the past few weeks we have been prototyping several different motor mounting options. Luckily we built a RepRap a few months ago which makes prototyping parts quick and easy. Here is one of the mount designs.
The motors would mount inside the chassis rather than in the wheels. The bearing will relieve some of the stresses being placed on the motors from the weight of the robot. The encoders sit inside this block so that they are protected from the elements.
We have ordered a set of these parts from the machine shop which should be here this week.