HI Will,
Not exactly what you ask, but I´ve been playing with a toy camera VF which "normal lens" FOV matches one of a 35 mm lens. This VF, if reversed, covers a little more than a 135mm lens FOV, then a small mask in the viewing side will do the trick.
If you have a 35mm VF, then it would be quite easy to mask it if you remember that a 35 covers about 63 deg. in the diagonal of the neg. and a 50 mm normal lens about 46 deg.
I used a SLR body and 35 and 135 mm lenses to make the checks, plus a measuring tape affixed to a white paper piece held in the wall ( I´ve measured FOV horizontally and vertically, marked the paper and then by trial and error cut the mask).
The mask can be done with a piece of polyestyrene painted matt black, and placed in front of the VF. Affixing it on the VF is another issue, as it depends on if the VF is round or square.
In your case, the mask should be placed in front of the VF so to limit the FOV available, mine is just the opposite.
Hope it helps!!
Cheers,
Ernesto