Maybe I'm a bit outdated here (it's been a while since I last posted, or for that matter browsed through the forums), but a while back someone here made a "gravity shutter" by building a box around the lens and having the foward side (perhaps made out of styrofoam) slide down in front of the lens. If I recall correctly, different sized horizontal gaps in the front side provided different shutter speeds (since they all fall at the same speed, etc..)

I thought that this might go well with the box camera theme.

Good luck,

André