Simon, I emailed this image to a club member who happens to be a fitter & turner. He managed to make me one in less than 1 week. As there is a bit of machining to do, I let him have the job. He's turned out, (sorry for pun), erect image finders from 1/2 bino's, extension tubes, 2" Crayford style focusers etc etc. He's a 'gem' to have in our club.
HTH. The E/Pc bracket, he made from spring steel, with a small steel plate welded inside at the bottom, for extra strength/thickness for the nut. It's sized/shaped to suit E/P you'll use most, & felt/rubber lined. Should fit others tho. It works well.