Another point - as a service tech, I find that the actual LCD panel surface on those alu-iMacs is VERY fragile, VERY difficult to keep clean. It's just not designed to be left open, and in my experience, collects every bit of dust in the area. The LCD surface is just not intended to be an outside surface.
Better answer, I think, is to find a product like a screen overlay, to cut down the reflections.