Fitting a meniscus lens onto a SLR such as a Hassleblad, Mamiya, Bronica, or Pentax is not as easy as putting one on a flat lensboard.

Without machining a special adapter, some ideas come to mind...

... Adapt a body cap.
This is probably the quickest solution if the body cap has bayonet lugs.
A slip-on style body cap may need some way to better secure it to the camera.
The lens tube can be screwed, glued, or duct taped to the cap.

... Use an extension tube.
The bayonet end of an extension tube set is an ideal connection.
It could even be configured for interchangeable lens barrels.
Machine each lens barrel to fit into the extension tube base unit.
A light slip fit with an O-ring might be just the ticket...

... Cannibalize a junk lens body.
Depending on design, the bayonet end portion might make a good adapter.
Break out the hacksaw...

For our 35mm friends, the ubiquitous "T" Mount would let them fit smaller meniscus lens tubes with minimal fuss...

No excuses now: Go Forth and fill the earth with soft focus photo's...