"He who marches out of step hears another drum." ~ Ken Kesey
I've done some looking into the metal halide grow lights and from what I can tell, these lamps require a few minutes to warm up to full output. Do all MH units use a shutter for exposure rather than a basic on/off/timer switch? Not sure I'm keen on trying to build a DIY light shutter.
My diy shutter is a piece of sheet metal i move to start exposure. I am also using a grow bulb. Basically, I have lots of airflow across the light and the glass, and manually remove and replace the shutter. It doesn't get hot and neither does the glass. You can make the shutter hinged or have it slide back and force or just lift it off and set it to the side. You don't need anything fancy and automated.