I have seen several dual fan blower assemblies for the Monoprice Ultimate / Wanhao D6, but the all seem to add unnecessary weight. This model is based on some of the ring ducts seen on other printers / extruders. It seems to do a good job without the extra weight.
The one modification I might make would be to slightly increase the diameter of the duct to give the heater block a bit more clearance so that you do not have to print it in ABS (in PLA the heat can cause the outer edge to sag and collide with your prints). Otherwise I am pretty happy with this part.
I have added a new design that increases the ring diameter to 70mm from 60mm. It turns out my concerns about the ring sagging and colliding with the prints were well founded after a couple of days of use. The larger design in PLA is continuing to work well even after a few days of use. The smaller diameter design is still available for those of you willing to print in higher temp material and who want the closer air flow. I continue to be pretty happy with this part.
A third design has been added. This one provides a full ring and a more square-ish ring profile. Both of these features are intended (and are working) to make the ring more stable when things get hot and keep the duct from drooping into the print. Also, to provide a larger cooling area, I removed the directional vents and added lots more outlets. This lets the unit cool an area much larger than just the point of extrusion and seems to work much better.
I will be grateful for your feedback and suggestions.