Hasselblad A16 back

I'm interested in picking up an A16 (6x4.5) back for the Hasselblad. How do you mask off the viewfinder for this back? Are you just using the grid lines on the focusing screen?

I'm assuming all other operations are normal as with the A12 back. Let me know if there's anything special I need to know about the A16 back.

And this back is horizontal only, right?

It is only horizontal. There are viewfinder screens which demark the 645 shooting area. I do not have the Hasselblad Compendium with me so I cannot look it up now.

I have one of those viewfinder screens. I can trace it out if you'd like - though it may take some time to put my hands on it. Then I'd suggest cutting it out of a sheet of matte acetate.