Print the plastic mount, I used ABS with no support material. There are some overhangs, but they are small and come out fine. PLA should also work, but I think the slight flexibility of ABS gives it the edge for this application.

Attach the top of the fan to the mount with 2 zip ties first. The 2 zip ties used to attach bottom of the fan must also go round the bottom threaded rod of the Printrbot, do this next. Finally wrap 2 zip ties around the middle nubs of the mount and the top threaded rod of the Printrbot - this is incredibly fiddly if the bot is already fully assembled but it is possible, try using pliers, tweezers and patience. Taking off the print bed would make it easier if you're really struggling.

I have provided the SCAD file so that you can change parameters to suit a different fan, but space is very tight down there and I haven't tested any other sizes so check your work!