I've some people mention that longer prints with PLA require active cooling. This duct is intended to replace one of the small plastic sides between the front and back acrylic plates for your MK5-6 style extruder.
The smallest fan I could find at the local electronics store was a 40mm fan. However, the front and back acrylic plates are only 35mm apart. This duct gives a mount for a 40mm fan, letting it blow air into a 40mm diameter cone, which narrows to 35mm, which goes to a 35mm wide duct which would point directly at the barrel of the extruder.
It's not really an easy print and wasn't exactly optimized to avoid overhangs. That said, I printed it with minimal problems.
EDIT 2011/05/026: I just installed this part this morning. It fits between the two sides perfectly, but I'll need to adjust some of the dimensions to allow the fan part to better fit.
- Install cooling duct in place of one of the two small acrylic pieces at the left or right of your extruder.
- Install 40mm fan.