Summary

This is yet another extruder fan surround for a 40mm fan. This designed to maximise airflow, and allow cooling from all angles. It's also modular and easy to print (even worked on my badly calibrated printer).
EDIT: A few problems have been identified (see below) which are being worked on...

Instructions

I designed this for fun when I was learning OpenSCAD, and finally made it, so thought I'd share it unfortuantely this means the SCAD file is a complete mess and the original STL output for fanTrunking(); from SCAD has holes but Netfabb (http://cloud.netfabb.com/) sorted it out.
If you're using the SCAD file, you'll need to comment out either the fanTrunking(); or the extruderSurroundPlacement(); as the extruderSurround is fairly delicate and I figure best printed separately (and they overlap and are not Z aligned).
You will probably want a fan running to reduce sagging (I wired the 40mm fan temporarily to the x-carriage while this was printing).
After printing you'll have to remove the front support to allow air in, but the inner support is optional.

Regarding Melting during use: Despite surrounding the entire extruder most of the inner wall (protected by Kapton tape) was OK for a number of prints. Even when it loosened (see below) it only melted the portion in direct contact with the extruder head. This was probably for two reasons: Firstly Slic3r did something funny with the inner support wall which meant it the wall printed as blobs linked with thin trails of plastic - effectively a wall with lots of tiny holes. I think this may have been because my wall thickness was thinner than the extruded filament. However I considered this a plus and left it intact. This was lined with Kapton, but I also have quite a few layers of Kapton holding air pockets to insulate the extruder, and the top 1/2 of the fan output is directed over the x-carriage and will also assist somewhat with the cooling - if yours is properly configured then you may want to leave the supports out (play with the extruderSurroundPlacement() module) in the SCAD file.
Unfortunately the outer wall wasn't insulated by Kapton, and after sitting just over an overheated bed without the fan on, it softened and distorted, which opened unwanted air gaps and reduced the apetures. However despite this it still works OK.
I did try filling the gaps with hot-melt glue, but the bed melted it and left a liquid glue dripping all overthe bed (DUH, who would have thought!!)
Another problem was that this softened out wall was a magnet for stray flakes of ABS - if anything didn't adhere to the bed correctly the outer wall was sure to catch it and rip it off completely. Thus a bit more than the 5mm clearance I tried would be a good idea.
I now plan to make a new version with kapton inside and out, and slightly smaller to improve clearances.

Yet another 360 degree extruder fan