I looked inside the viewfinder. Man, you couldn't build something yourself that nice! Stepped sides, flat black, masked holes, nice...
Another nice thing to hang on to is the flip-up rangefinders from the now-cheap 100-200-300 series Polaroid packfilm cameras. I saw a mod with a bolt, two nuts & a calibrated scale glued on...the screw moves the external actuator & you get your indication on the scale aligned with the actuator arm. Not sure how well it mounts on anything, but can be a hand-held accessory.