4W4K3 said:Is it acrylic? If so drilling it can be hard, acrylic cracks pretty easy.
What I would do is drill a circle about the size of the fan blade's circumference. This will let in a WHOLE lot more air than the small drilled holes, and is also alot easier. Then, just match up 4 holes with your fan's mounting holes and drill them on the outside of the circle you just drilled.