I printed a Toranado extruder for my 3d printer and I wanted quiet 40mm fan to cool the E3D V6 hot end instead of the very loud stock 30mm fan. I tried many of the existing ones on Thingiverse, but there were screw collision issues or thin wall issues on all I tried. So, I designed one with a slightly rotated 40mm fan. That let me easily get to all the holes for both the 30 and 40mm fans and still have good wall thickness.
The Toranado extruder doesn't allow for much extra width outside of the size of a 30mm fan, so the 30mm fan side has an outside dimension of 30x30mm.
I use this Noctua 40x10mm fan with my E3D. It's incredibly quiet and works great: http://amzn.to/24cGcb1
It needs supports for the underside of the 30m bolt holes. Other than that, just about any print settings should work. I printed mine in PETG at 0.28mm layer thickness, 4 bottom layers, 5 top layers and 3 perimeter layers.