Replace the Cradle Ring with the Camera Bulkhead

[Joedang]

The bulkhead from the camera ring, although requiring more parts, is way easier to manufacture than the current cradle ring. (The cradle ring requires a 2.5 axis mill and a rotary table, while the bulkhead only requires a 2.5 axis mill.) We should replace the cradle rings with a pair of bulkheads.

While you're (I'm?) at it, the length of the cradle cage will need to be changed, since the bulkheads sit at a different height from the mating surfaces of the module. Also, the acrylic plates that the rails sit on should be replaced with plywood. (Aaand there should be a drawing indicating the major grain direction of the cradle cage pieces. That should be its own issue, but bleh.)

[Joedang]

Done. See 87c7b8f.