DIY Geared Focus?

I've got a couple of 500mm mirror lenses, which are as widely acknowledged, difficult to focus.

I'm looking for DIY plans to build a follow focus TYPE device to help me move the focus rings in very small increments.

The problem that I've run into is that while I've found a lot of suggestions, almost all of them are for video, and rely upon the sort of video dual rod mount which I don't need and will never use for its intended purpose. I literally have no intention of EVER recording video.

The one or two exceptions I've seen seem to be designed for mirror lenses different from mine.

What I'm looking for is something relatively simple that's designed to attach either to the tripod socket of the camera or the auxiliary threaded hole on my "L" bracket.

I have a moderate selection of tools, but no ability to bend heavy metal parts, weld, etc.

THis...
https://www.bhphotovid​eo.com …wE&is=REG&m=Y&s​ku=1000317

EightEleven wrote in post #18409896
THis...
https://www.bhphotovid​eo.com …wE&is=REG&m=Y&s​ku=1000317

If it actually had a gear or roller mechanism to do the adjustments, it'd be exactly what I needed.

The problem with the lens is that depth of field is small and focus is sticky and coarse enough that it's tough to make small adjustments.

Functionally, this device isn't materially different from the one I've made using a hose clamp and a 1/4" - 20 eye bolt. The advantage that this device has is that if you're doing video, you can set up presets. Unfortunately, that's not something I will ever need.

Thanks anyway.

If I'm not mistaken, by saying, "for video", you mean to say, something like follow focus, right? I did also research about gear focus, but to no avail, none of them relevant to the said keywords appeared — results showed follow focus. For follow focus, I've seen a lot of DIY's use gear rings and a magnifying glass, which has its glass popped out.