Surly, it occurs to me that with a process lens you probably aren't going to need high shutter speeds -- a rotary shutter (like in your Holga) with B capability, operated by a locking cable release to give walk-away time exposures, would probably do very nicely. You'd get one "instant" speed (potentially adjustable by swapping a spring or adding/subtracting mass on the rotary disk), and B, and you could make the thing easily from hobby brass and music wire from the local model airplane shop. The shutter need not even be huge; it only has to accommodate an opening the size of the largest stop in the lenses, if you can somehow mount it between the groups just ahead of or behind the aperture (should be at least as large as the back element if you mount the shutter behind the lens, though). I've seen these in old roll film box cameras that had disks as much as three inches across, giving about 1/25.