I am looking to purchase a 6X4.5 A16 film back for 120 film for the Hasselblad including the viewfinder 6x4.5 mask. My plastic Fuji 645 "professional" folder's rangfinder just died so it's time to return to my tried and true Hassy. Landscape (horizontal) versions only.
If it helps, you can use a gridded focus screen, the upper and lower grid lines coincide with the 645 frame.
Backs without the mask are far more common, as you've probably discovered.
If that isn't a workable solution, I'd look to making my own mask from a piece of exposed film, or similar.
For me, since I prefer the gridded screen anyway, the freedom of not keeping track of the mask is a nice plus.