If that solar panel doesn't work out for you and you're up for trying something a little different you could power a small fan with the heat generated by the halide. It's called thermoelectric and all you need is a Peltier plate which you'll mount to a hot spot on your light fixture and it will generate a small amount of electricity, should be enough to power a small computer fan. The whole system will probably be smaller and less noticeable.
A Peltier plate looks like this:
You may need a heat sink on the one side to increase the heat flow through the plate.