I can't really suggest using a screw. Even if you get a stainless steel screw, odds are, it isn't 100% stainless. A good test of this is to hold a strong magnet up to it. If it sticks, it's not 100% stainless, if it doesn't stick it is 100% stainless.
I use aquarium silicon to attach it to a piece of thin slate, bury the slate in the gravel so the slate is actually resting on the bottom glass, and viola! You don't see the slate, or the silicone holding it as the siliconed part is hidden under gravel!