This is another cooling design, inspired by (but not directly based on) thing 5756.
I wanted to use Iwo's design, but I just upgraded to a MakerGear stepper plastruder on my Thing-o-Matic, and there wasn't enough clearance around the extruder nozzle to mount any fan ducts on the top side of the Z-stage. So, this design is a circular cooling nozzle that mounts around the extruder nozzle, just under the Z-stage.
It is fed by two 1/4" PVC hoses, attached to a fan mount that sits on top of the Z-stage, near the rear of the bot. This mount holds a 40mm fan salvaged from an old graphics card.
The overall airflow is very subtle- it's detectable with your hand, but just barely. But it makes a huge difference in print quality for small parts. In the attached image of two single-wall calibration boxes, both boxes were printed consecutively using the same g-code. The only difference is that one box had the cooling fan intake blocked.
This design is in parametric OpenSCAD, so it should be fairly easy to adapt to other nozzle types or fan sizes.
Note: This adds some extra hardware to the underside of your Z-stage. Make sure there will be clearance with anything you have sticking up from your build platform. I had to remove the nozzle wiper from my ABP. (The wiper wasn't working very well with my MakerGear nozzle anyway, so no big loss...)