Instead of DIY I´d look out for a Voigtlaender "Kontursucher" - they are not that rare nor that expensive and they apply a stunningly simple, yet clever principle.

Instead of explaining it, look for yourself ;-) - it´s in german, but you should understand the principle by looking at the images:

You mount the finder on the camera and hold it front of your face - keep both eyes open. The left eye will see the situation, the right one will "catch" the viewfinder frame while the brain makes a composite image of the both.