Flap Vent

I (Adrian Bowyer) installed a big (300mm) fan in one of RepRapPro Ltd's laboratories and so needed a flap vent to go on the outside wall. I could buy 100mm ones and 150mm ones, but nothing bigger.
So I designed one and printed it on a RepRapPro Mono Mendel.
It is parametric, so you can just set the diameter of the fan and the number of flaps that you want at the top of the OpenSCAD file, and it'll give you a vent to match.
The OpenSCAD program will chop the frame into quarters, and the fl