The standard way is with a focus screen adapter, it mounts in place of a film magazine and has a focusing screen, it will also accept any viewfinder. Basically it places a focus screen at the same plane as the film.
Hasselblad Focusing screen adapter - 41057Hasselblad Focusing screen adapter - 41057 (new style); in 1997 ins Programm genommen für Flexbody, ArcBody, SWC Eingebaute „Wasserwaage (Bubble Level)“, grid screen and split image centreSchöner, sauberer…
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It was more or less primarily intended as a way to get through the lens viewing with the super-wide models, but it can be used on any body.