Next using a straightedge, draw a line from the opposite corners of each corner of the marks you made from the fan placement. You should make an X, the center of this X is the center of your circle. Using a compass or Scribe set to 1.5" (or 40mm for those metricly inclined) draw a circle using the center of the X as the center point, giving you a 3" or 80mm circle. Repeat for the other fan.
Using a dremel with a circle cutter set for 1.5" radius, cut out the cirlcle. It might help to use a automatic center punch to mark the pivot point to have a deeper hole to move around on.
Once this is complete you can use some 400 grit sand paper or a fine toothed file to clean up and square the edges if need. Note: I freehanded the first hole for the hell of it since I thought I was a badass. It looks like crap so use the circle cutter.
The Switch hole is cut with a cut-off wheel and the power cord hole is drilled. Using slow speeds is a MUST or you will crack your work of art and it will look crappy. Slow Speeds you understand? good. When drilling, drill with a wooden backing under the plexi and keep pressure on the work so there is no tear out, i.e. cracking!!
Test fit the switch and the cord keeper, since it is much more difficult to make any modifications after we make the bends.
Lay the fans over the holes we just cut and mark the corner hole positions with a nail punch or pen. Drill these out the same way as you did the powercord hole. Clean up any remaining rough edges and lets get ready to start forming this magnificant bastard.