65 degrees diagonal on 6x6 is about a 65mm focal length. Is that what you have? It would be simpler to say what focal length and format you are using since finders are usually marked/sold by the focal length, not by degrees of field of view. It's also about a 28mm focal length on 24x24 film (cropped 35mm).
There is a 65mm shoe-mount aux viewfinder for the Mamiya Press. It has a field of view for 6x9 and little arrows to indicate a 6x7 crop. You could add a mask in front (or just tape off the edges) to indicate 6x6. The mask will probably be a little out of focus but may be acceptable. You could also do the same thing by masking a finder intended for a 28mm lens on standard 35mm (24x36) down to a square.
If you can find a wire-frame sports finder for 6x6 that adjusts to match a 65mm focal length, that would also work. So far as I know, the Hasselblad sports finder does not go that wide.
There are people who will 3D-print frame finders (no optics) for various combinations of focal length and format, like cameradactyl.com.