you might look into buying a shutter, like a copal or prontar or betax, alphax or ...
and "front mount it " to your lens. i have never done this, but have heard
of folks front mounting shutters, and from what i gather it isn't as expensive
as having it mounted "in a shutter" because there are not shims, or crazy machining or critical spacing involved.
it mounts on like a big filter, and you can take it off
(or not? ) when you want to use it as a barrel.
not quite sure if there would be vignetting ... as i said, never done it, but steve grimes &co
suggested it to me at one time, so i guess it is a viable option ...
ps. there is always the jim galli shutter or the top hat / lens cap method as well