[Chris is me]

I know it's popular on CD these days, but I don't think the gearbox in tube setup here is good. You end up needing a wider tube than necessary to pull it off, adding weight across the entire span of the tube, but the tube is substantially weakened right where strength and rigidity is needed the most - at the wheels. Switching to a larger tube to avoid these big pockets just adds more weight to the setup.

You already are making something out of plate to mount the CIMs to the tube - if you really want as few parts as possible, just make one gearbox half out of plate and use the tube side as the other half of the gearbox. There are reasons not to do this (mainly design flexibility if you change your mind about gearing, admittedly not common) but it does reduce part count and would be similar to your current setup with less weight spent on a tube that's wider than you need.