I was looking at extruder fan grilles for my Maker Select, and considered a few, but they all seemed to have one or another near-fatal flaw. This is my attempt to combine ideas from all the grilles I looked at and solve their flaws.
The hexagon pattern is specifically made with ribs .8mm wide to ensure two nozzle passes using the stock .4mm nozzle, and to restrict a minimum of airflow. Orignially the hex pattern went all the way across, but then I remembered why we use these: to protect the fan while giving us a place to push the carriage. This is where the central disk came in.
The next problem to solve was mounting it. Other designs have their own well for the screw head, which can affect how much thread engages with the motor body. M3x55 or M3x60 screws can he hard to find, so I avoided that possibility altogether by just making the holes for the screws to pass through.
Some other designs use slotted pins. I printed two iterations with slotted pins, and the pins broke off of both of them. Another option I considered and discarded was to integrate small clips into the design. I considered the bottom side a hot zone unsuitable for PLA, and the heat sink severely limits clip size. So that was out.
The only mounting soliution left was press fit. That is, solid pins that are just oversize for an M3 hole to hold the grille in place by friction. The pin diameters match the screw size at the ends, but bulge out to .25mm more than the screw size in the middle. The narrow end helps with insertion, and because some fans have head wells on one or both sizes of the fan chassis, the only reliable hold area was the middle, hence the bulge.
The slots around the back edge are to aid in removing the grille if the fit is tight. Insert a flat screwdriver and twist so that the side of the slot near the pin gets pulled away.
Variation in fan design is why there are two versions. Some fans have M3 holes, other have M4. Each file is named for the screw size that matches the pins. If you have a stock fan, use the M3 version.
The M3 grille I printed fits with barely any force required, yet doesn't rattle and hasn't fallen off.
EDIT: The 'b' files add .05mm diameter to the thick part of the pins (to nominal + .3mm), which should make for a tighter fit.
Print with 3 shells (1.2mm thickness) and 1.2mm top/bottom to match. The top/bottom thickness gives it strength to prevent it bending inward to contact the fan hub.
After printing, it's likely that the pins will be messy. Carefully scrape any excess material off the pins. Make sure they are smooth and glob-free before mounting.