This cutaway of the CAD model shows the lens (transparent green), the body (yellow), lens hood (pink), and a filter holder (blue). The lens screws in to the body using the helical channel cut into it, and is held in place simply by friction. It can be screwed in or out to put it at the correct position for focusing at infinity (i.e. when the helicoid is fully collapsed).

The filter holder is a separate piece so that installing, removing, and adjusting the lens within the body is easy to do. The screw threads on the body are M42, which screws into a focusing helicoid, which then screws into a M42 to E mount (Sony). The last two were purchased instead of being printed, as he wanted a metal mount, and it would be very difficult to get a nice smooth helicoid out of printed parts.

I tried printing the filter holder twice, but since I didn't know the tolerance for plastic to plastic threading, neither fit. He's not terribly interested in putting a filter on it, so although I could print one now that would fit, I'll wait until there's a need.

Finally, this is actually the second iteration of my design for this. The 'mk I' had no lens or filter mount, and was mostly to test for fit and feasibility. This 'mk II' version is also significantly stronger, as I used thicker walls for the body.