I have a box of lens elements from various failed lens repairing escapades over the years and I did a bit of Google research.
I found the simple Gallilean viewfinder which is basically a telescope but I found I couldn't get a wider than 'normal' view with just a positive and negative lens.
After playing around with two lenses, I tried three. A large positive at the front, a negative just behind it and a smaller positive lens at the eyepiece.
I found the spacing of the front and middle elements was not very critical but the middle to eyepiece element spacing had to be correct to get a perfectly focussed image. I fixed it to suit my vision corrected with glasses so I assume it will also suit an eye with perfect vision.
A more sophisticated version would have this variable to suit different people's sight but I will keep mine fixed.
In between the front and middle elements is a black polyester disc with a rectangle cut out of it. It is actually slightly pin cushioned in shape to compensate for the distortion of the lens. When you look through it, it gives a really good image with a clear rectangular mask - much better than that picture taken by holding it in front of a digi-compact on macro.
I will probably re-make the housing for the viewfinder as the one shown is lots of bits of CNC cut PVC stuck together. I am going to try to find some plastic pipe with an internal diameter to match the front element. I will then fit a hot shoe from an old flash (or make one) onto the base and put an accessory shoe on the top of the camera.
I have not made much progress recently but I am getting ready to cut the oak for the body.