The Biogon has an vertical FOV equivalent to 16mm in the 135 format. So you could use the 15mm Voigtlander or the 16mm VF for the Contax and visualize a square crop. There is also the Fotoman VF which accepts custom masks. It was inexpensive when I got mine but it doesn't look like it's available anymore.
You would need some sort of cold shoe extension to retract the VF. Not too hard to do.
When my sister and I were about 5 yrs old we would walk a farm field path to the neighbors house everyday to get a peppermint from the neighbor lady we called grandma. She had a pet goose that used to hide behind her chicken coup and wait for my sister and I to head for home. As soon as we got just past the chicken coup he'd come out flopping his wings and honking like crazy. He'd hit that field path running and flopping full throttle. We didn't stand a chance of out running him and he'd bite our butts until we reached our property and then he'd head back home. He'd never attack us going there, but would nail us on the way back. Yes, you could certainly feel the bite through our thin cotton shorts. I think we made his day. Oh, I think it made grandpa and grandma neighbors day too. At least he'd let us get the peppermint.