I've done this before with a dremel, though the hole saw idea sounds good too. With a dremel: First draw a circle with a fan gaurd of the size fan you will use, using the fan gaurd like a template draw trace around the edge with a marker or grease pencil. When cutting go slow and do small bits at a time, plexi will melt a little. Then use the fan gaurd also to mark the screw holes. Easy that way.