Main body as viewed from the bottom. The two rows of bricks on the bottom give the model a backward tilt. The slot allows the wheel to pass through.
The back half of the floor of the hopper. It sets on hinge bricks on the left side, allowing it to slope from left to right. This combined with the tilt from the base
causes to balls to congregate in the back right corner.
This is the front half of the floor.
The two spoke designs. The first attaches to the hub. After I generated the instructions I found a way to use a single spoke design instead of two. I was able to use two fewer part types that way. The main image shows the newer version.
The trivial drive train. The worm gear box came in a LEGO Creator set, but a more common Technic worm gear box should work almost as well. With 16 slots on the wheel, it only needs to turn at 4 rpm to meet the GBC specification of 30 balls per 30 seconds. Even with the 24 to 1 worm gear reduction, full speed is too fast. The downside of using a worm gear is that the wheel can't be turned by hand. The two spur gears on the drive axle allow one turn turn it by hand, rotating the wheel when necessary.
The front wall of the hopper. It had to be mounted at a forward angle to compensate for the tilt of the base.
The last step of the wheel assembly and the completed wheel. Note that you don't really need all 16 cells of the wheel to make a working GBC module. Building just four of them is probably enough.