This is a derivative of my own prior extruder barrel cooling duct. ( http://www.thingiverse.com/thing:8461 )It is optimized for a 40mm fan, which won't fit inside the 35mm gap between the two large sides of a standard MK5-6 extruder. I find that PLA prints much better with active cooling directly on the barrel itself.The section STL is for a replacement side to the extruder. It is functionally identical to the original small acrylic piece that fits there - except that it has a big rectangular hole to allow additional ventilation.I've of course included the OpenSCAD design files, but I didn't try to make the fan mount terribly parametric. There are some variables you could set, like thickness to all parts, but it's not fully parametric. If you have a different fan from mine (40x40x26mm) then you're going to want to redesign the fan mount anyhow.The reason for the DXF of the front acrylic panel was so that I could essentially create a virtual extruder holder for making sure everything aligned. If you uncomment the section at the top, you can see it too. ;)