If you've printed my adaptor to use 35mm film in a Hasselblad A24 magazine you will find this viewfinder frame a useful aid when taking photos. It masks to a 2.3:1 aspect ratio and should be orientated vertically under the viewfinder screen in any of the 500-series cameras.
A viewfinder mask is thin and needs to be strong, flat and dimensionally stable so I printed it in black Prusament PC Blend. This has been excellent but you might find one of the easier filaments like PLA will also work (since I only needed one of these I haven't experimented).
There are more details about the photographic side of this project on my blog
The author hasn't provided the model origin yet.