I've attached a sketch of the prototype "double motor module" that I'm building to test the feasibility of using these cheap brushless RC motors.
I've tried to make it easy to manufacture, so the main body is just a length of 2" x 3" x 10g alloy box section. This forms the mount for both motors, plus the plenum chamber for the cooling system. The motors have a cooling air "intake" around the mounting face and can exhaust hot air out through the vents in the end face of the rotor bell.
The central output shaft is supported by a simple bearing holder machined from a short length of 1 1/2" alloy bar, with two bearings pressed in at either end. Flange bearings could be used for those who want to splash out another £15 or so!
The module can be bolted down with a couple of through bolts either side of the output shaft, which are probably best fitted with internal anti-crush sleeves.
I may look at fitting the fans inside the box section on small bulkheads, with intake holes drilled in the top face of the box.
By fitting a longer shaft, two or three of these modules could be stacked together. If intended for driving a chain, then it might be an idea to fit the chain sprocket between two modules,
Jeremy