Ive done it, but no detailed pics up online currently.
The most important thing are the gears. You can get them from stevens international. Check the local hobby shops, as they may carry them in stock. (mine did)
Unlike the design posted, i used a gear collar method: I popped a wooden dowel with a hole drilled in the center, to the side of each gear. Screwed on with some small woodscrews. Then drilled a small hole in the wood collar, and inserted a set-screw into it, to hold it to the shaft securely.
The hardest part is the centering. I probably should have used torsion springs like the original.. but opted for the easy way out.
The unit also needs rubber bumpers, and a very strong shell.. because a good slam could rip it apart pretty easily.