Would a drop shutter be the way to go? If you could put together a light tight box which could take various slotted pieces of wood or aluminium at the front, they could be actuated by pulling a pin. Slot would drop across the lens, after which the shutter would be light tight again. Varying sized slots would give you different shutter speeds.
I have seen these used on a TV program about historical processes and they seemed to work well, gravity being nothing if not consistent. This link shows the design under guillotine shutters although they describe it as rubber band or spring powered going across the lens, I think it might simpler and more consistent dropping down with gravity.
http://www.earlyphotography.co.uk/site/shuttern.html
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