Again, I could spend £100+ on a viewfinder or I could make my own from some spare lens elements.
After some Googling to learn a bit about optical design, I started playing around with some lenses. If I understand correctly, using a simple negative lens at the eypiece and a positive lens at the other end, I can make a Galilean viewer (or a very simple telescope). The problem with this is that I can only get life size or bigger. i.e. positive magnification.
After a bit of experimentation, I found that a positive, negative positive combination gave me the view I wanted with about an 85 degree view once masked to a rectangle.
The pictures here show the elements mounted in some prototype mounts. I am going to make something better, this is just to see if it works.
The second picture is what it looks like if you hold it up to one of those non-film cameras on macro mode. The actual view with your eye is much better than this and the rectangular mask is crisp and clear but it does show the wide angle of view.