When you put the Rovers gearbox cover on, it will press on the frame of the gearbox and hold the assembly in position nicely. The rear of the gear box protrudes through the back of the rover and this is held snugly as well. I used a child's toy plastic drinking straw over the rear wheels axle to hold the axle shafts together when I was finished. You may have to try a few different diameter tubes, because I am sure I got lucky on the straw diameter. I would think a rubber hose, like a windshield washer hose would be close to the inside diameter you would need.
Hobby shops sell shaft collars and Universal joints with setscrews as well.
Be careful here because there is not a lot of clearance between the axle and the gearbox. Final assembled Rover. At this time I am using the wired controller to control the Rover. Future plans are to install a Basic Stamp II controller. All rights reserved.