I wanted a 50mm right-angle finder, but didn’t fancy paying the £90 to £100 they seem to cost, so after someone mentioned it was possible I decided to make my own. I picked up a 50mm binocular objective, cheap diagonal and 25mm eyepiece from Astroboot for about £15 and rummaged through stuff I had lying around for the rest. The mount came from (I think) a dead red dot finder and most of the rest is lengths of various diameters of plastic pipe I had lying about.
This one is very much a “proof of concept” and was useful to make because it brought home just how much of the focal length could be taken up by the diagonal (as much as 60 or 70mm). I intend to make another (though straight through this time, so I can potentially use it as a finder-guider) and will cover the build step-by-step now I know what I’m doing. For the moment however, here are a couple of photos of my completed (but as yet unpainted) finder: