There have been other fan shrouds created, but I wanted something to focus most of the airflow onto the heatsink while trying to absolutely minimize airflow disturbances in the nozzle/heater block area.
Drawing air directly over a hot, just-printed ABS layer is a recipe for warping and delamination!
This shroud uses the stock heatsink, and fits as snugly as possible while still allowing clearance for the heating cartridge and thermistor wires. It will clear the carriage when installing, but you must carefully route the heater cartridge wires against the chamfered corner of the shroud as shown in the photos. You also might want to apply kapton or metal tape to several areas around the extruder metal housing where there are air leaks.
UPDATE: New pictures added to show location of cutouts for proper routing of thermistor wires. Also pictures to show air leak locations that should be sealed!
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After experimenting with the fan blowing both ways, it has become clear that it MUST be mounted to suck air up through the shroud - NOT blowing air into it. If blowing air in, there is too much airflow around the hotend and the part. This may cause difficultly in getting the hotend up to temperature. It may also aggravate part warpage.
I have also applied a piece of Kapton tape to cover the gap between the bottom of the duct on the shroud and the carriage to try to eliminate airflow in the hotend area even more. I strongly advise doing this - it works.
Thermal heatsink compound applied to the heatsink (and in my case to the extra thermal barrel heatsink) greatly improves the heat transfer and significantly improves the potential for printing PLA.
I used DOW 340 Silicone heatsink compound, but if you have some Arctic Silver lying around, use that!
I sliced this with Slic3r and set the support material overhang threshold to 74 degrees. This will leave the uppermost layers of the duct unsupported, but it should bridge OK. I also used a 3mm brim around the part.
Note the small cutouts with radiused edges I've put on the right side for routing the thermistor wires.
I tried to size the fan mounting holes so that the original screws could be used and they would self-thread into ABS.