Adjustable Focus
Although functional, the preceding lens still doesn't have a mechanism for adjustable focus. Let's look at a few approaches to solve that problem.
The first approach is to "recycle" the helical focuser from an old lens. Mounts and filter threads can be attached with Super Glue if necessary. This lens also includes an old iris diaphragm I found for $3 at a local surplus shop. Attaching rings from junk filters to both the focuser and the diaphragm will allow you to use them in an interchangeable system. This lens uses a 191 mm achromat ($9 from SurplusShed).