This is just an amalgam of many great ideas, none of which are mine. I needed something a little more direct than sndlover's design (http://www.thingiverse.com/thing:1336355) with the finesse of thruit00's design (http://www.thingiverse.com/thing:537918), so I came up with this monstrosity. YMMV.
Three versions. The original is a remix of a remix of thruit00's design for only one extruder. The second is the same, but reversed for a better view of the extruder tip. The third includes both side for (in theory) maximum airflow. Combine with a high-output fan and you should be good to go.
If there's any interest, I'll make this work available on Tinkercad.
Note that this should print fine at 0.300 layer height with no supports, but you may need to tinker with the number of top layers and/or shells depending on your printer's performance.