[RC-Monster Mike]

Cool looking tranny. I understand that some people want the gearbox enclosed, but there really is no need to do it - and any lube you think you may put on the gears will fly off regardless of if there is an enclosure or not. One huge advantage of an open gearbox when using the slipperential is the ability to adjust the slipper without taking it all apart to get to the adjustment nut. Another possible advantage I can think of is the occasional situation where the pinion set screw loosens. It can loosen to the point of actually coming out of the pinion - in an open design, it just flies away. In the closed box, it is likely to get into the gear mesh and cause carnage.
Also, I think the prototype will be fine with the rapid prototyping, but the motor has some weight, and there will be a substantial amount of torque on the assembly as the motor generates the needed power - aluminum is probably the best bet for a rigid tranny and sound gear mesh. Looks to be a cool project and I am excited to see it completed. :)