Intro: Printrbot Fan Shroud Spacers/Washers
If you ever have a problem with your M3 Fan Screws/Bolts being too long, this will solve your problem. These 1mm, 2mm and 3mm spacers will take up the extra length. Just print them out and place them between the fan shroud and your fan holder on the frame and those too long screws will work. The spaces come in 3 sizes so you can print out the size you want, or multiples of a size to stack them nicely.
I recommend a minimum of 25% fill as they're fragile enough in PLA even with a high level of fill.