The straps I have mount with clips to posts that are on the topside of the Rollei. This is true whether or not the leather case encloses the camera.

Since I almost always use the Rollei with a tripod, I suspect I'll just carry it in one the several packs of various descriptions I have for this very purpose. Not as spontaneous, but using MF is as much a thought process as a mechanical image-capturing process anyway.