When I first started adapting lenses without focus capabilities around 2008 on the Canon bodies, focus helicoids were prohibitively expensive, and there were very few choices as well. So I just used lens barrels from existing, and often broken lenses. This worked somewhat, but never satisfactory. One of the problems is the focus throw is usually very short, which mean you can't focus very close at all. The other limiting factors was the the rear of the lens usually had too small an opening for long lenses, and this would cause dark corners. Further more, these adaptations were one of a kind; one lens barrel for one lens. Not a big deal if you only have a few lenses but that could quickly get into problems when you have dozens of lenses to adapt.
Early attempt samples
Due to the number of lenses I have to adapt, it's not feasible for me to have dedicated tubes/helicoids for each of my lenses, so I needed a modular system that I can quickly re-use the tubes and helicoid for different lenses. For the next part, we will talk about a modular system.