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 flaps in half, for easy printing of large versions.

Update one year later. For all who worry about PLA degrading, this vent has been out in the ice, rain and sun for a year and is as good as new.

Print the parts.
If you have printed the flaps in two halves, glue them together (see the picture). Use cyanoacrylate (superglue). Cover one of the mating faces with a thin film of glue, then put the flat faces of the flaps down on a polythene bag on a flat surface (cyanoacrylate won't stick to polythene). Clamp the webs gently together with a small G clamp, and put an M3 screw and nut in the hole at the top to make sure that's aligned. Weigh the two halves down flat (two cans of fruit work well).
When the glue is set, take the screws out and put 3mm metal rods through that stick out about 10mm either ends of the flaps. Use something that won't corrode in the weather, like galvanised or stainless steel. The rods should be gripped and not slide in the flaps, but if they are a bit lose dribble a bit of cyanoacrylate on them where they go through the holes in the flaps to secure them.
Use two 3mm screws to attach the left top and bottom halves of the frame together, and the right top and bottom halves of the frame together.
Put the two halves of the frame together top and bottom with two more 3mm screws, with the flaps and their rods mounted in their holes in the frame.
Screw the resulting flap vent to the wall where the big hole from your fan is.