Extruder cooling duct

This is a minimalistic improvement on the extruder cooling.
I did not want to completely replace the standard setup, but saw there was room for improvement in the existing setup. The air is now forced over the extruder cooler and cannot leak out to the side or bottom. It also is less likely to cool the hot end.
Since it's close to the hot end, I created it from PETG. But with the cool air constantly flowing over it, it should not get hot at all.