I am thinking about buying a vintage barrel lens for my 4x5, preferably one with a bit of haze, and shoot it wide open to get "dreamy & soft" images. Barrel lenses don't have a shutter and if I use the the "lens cap shutter" method, I need to have exposures above one second to get realizable exposure times.

The problem is, of course, how do I shoot at wide apertures (f/4.5) and long exposures? I guess a +10 stop ND filter would do the trick, but most vintage barrel lenses don't have a filter thread.

Any ideas how I can shoot long exposures at wide aperture? Maybe glue a Lee filter holder to the barrel or use a circular filter with gaffer tape?