I was trying to recall the name of the Leitz finder when I saw that Richiedcruz had given it to you. You could make an imitation, or else you could follow Wlewisiii's procedure. Such finders are a little difficult to use, since the eye must be precisely centred. The Leitz one I saw decades ago, though, was good for several focal lengths. That too should not be difficult to copy: you just change the front mask. I cannot recall if it could be adjusted for parallax. There have been folding finders made with lenses, essentially the same as "ordinary" accessory finders except that they had no bodies. Since an "ordinary" optical finder is not much larger than a folding sports finder, or even a variant with lenses, the only advantage that the folding kind offers is that it can accommodate more than one focal length: but with eye-centring and parallax problems. Wlewisiii, do you mean "shoe" or "foot", that hoof sort of anatomical extremity?