Look at this site:
http://msfastro.net/articles/finder_guidescope/
I did something very similar with it on a spare finder scope I had kicking around, however instead of making a dedicated insert, I just got a 2" to 1.25" adapter, unscrewed the lens out of the finder scope, and turned down the adapter to fit, which from memory was just under 26 thou on the lathe. I made the fit so snug, it gave just enough resistance that with a slight tap of the hand on it, to make it secure. You won't actually need to do any thread cutting, nor will it affect the thread in the tube.
The beauty of doing it that way, was, at any time I could simply remove it, and screw back in the original lens. I found that the focus point was slightly different than described in that article, but the cmos chip from memory was well within the travel of the tube's limits.
I also found I didn't have to unscrew and remove the locking ring as described, rather, just screw that entire end (lens and locking ring) further onto the tube.
Hope that helps
Mark
PS: Having done all that - bought a ST80 2 weeks later....go figure. Not that it didn't work wonderfully, I found I wanted to use the finderscope visually for it's intended purpose. You could easily auto-guide from the above method though.
