Just looking at your proto design, could the back from a Calumet 4x5 (CC-400?) be modified to fit? I don't know if the round rail they use is close to the size you are using.

For that matter, remove the front standard from the sliding base of a CC-400, add a flat plate and mount your lens supports to that and keep the existing back and rail. Attach the bellows to your adapter plate for the lens and your done. That would cut the amount of machining down to two lens support plates, a bottom plate, and the bellows attachment.