After doing a lot of reading and a bit of testing this fan mount was the best solution I have yet found for the Printrbot LC.
I tried the http://www.thingiverse.com/thing:21462 fan mount, but it acted as a shim to raise the extruder. I had to compensate my lowering the z axis so low that it hit the hold downs for the glass. I ended up losing almost 3 inches on the Y axis. No good. I tested the part to see how it did at cooling. It cooled really well, but noted that I lost almost 15 deg C on the heated platform. :-/ Not wanting to find another solution for my heated platform I kept looking.
Then I found http://www.thingiverse.com/thing:15899. So I tested placing the fan on the side where it would still move up and down with the extruder. The headed platform only lost 3deg C and the parts printed really well.
This part completely replaces the wood one the Printrbot came with. I did this so I could use the same screws. The hardware store near my house doesn't carry 6-32x1 3/4 screws. 2 inch would hit the drive belt.
I've attached an image of two gears I printed. Both at .2mm layer height. The gear on the left is with the fan on and the gear on the right is with the fan off.
Just print it out, I used .4 fill with Slic3r. It matters how you install the part. One direction will place the fan too close to the extruder carrier and will stop it prematurly. I have pictures of it as I did this first. Then I realized the part could be flipped and the carrier had the needed clearance. This is for a 40mm fan. I got mine from some spare parts I took off the chip set of a discarded computer.