That Finderscope Mount

 Finally complete and installed.  This picture shows the broken and repaired aluminum version on the right.  The slight deformities where the bones did not knit well should be obvious.  The interior dimensions of the rings are identical; it may not be obvious because the broken one is farther back and white objects always show bigger.  (Yes, human optical systems are racist.)

This is the setup I used to put the screw holes 120 degrees apart:

This is the dovetail base with 75 degree angle:

And this is how I cut that angle:

The thumbscrews in the rings were going to be M3x0.5, just like the original, but I used the wrong drill bit on the holes in the rings and had to go with 8-32 instead.

Installed:

I spent a bit of time at the end discovering that while acetal is not Teflon, it is very slippery.  Through holes are always slightly larger than the threaded holes into which the screw goes.  A little larger on the through holes reduces the chance that your through holes and threaded holes are not perfectly aligned, but with acetal, it also means you really cannot get a solid grip that does not allow some slip.  So I cut a .1" deep slot at each end of the piece that holds the rings.  Now it is not only held in position by the clamping force of the screws but also by the slot that holds the rings.  

This also solved another problem: the 8-32 SHCS screws holding the rings to the base were just slightly too long.  A 3/8" length was not enough to get a solid grip.  That .1" deep slot meant that 3/8" length is just right.  They have a good grip but do not protrude into the rings.

The original mount was held in place by an M3x0.5 thumbscrew using friction along the edge of the dovetail alone.  This is bad from a fore and aft point of view and not very solid hold left to right.  My original solution was drill a hole into the original base for the set screw to lock into.  It did not work well because the set screw was too short to take full advantage of that hole so I ended up adding another set screw (a 6-32) to lock it more firmly.

The new one uses a longer M3x0.5 setscrew that goes all the way into the hole.  This is needed because the base is a little too narrow to otherwise achieve a tight fit.  I may remake this whole mount in aluminum at some point, or just remake the base part in acetal.  Everything is held together by 8-32 screws so that will not be a problem.