Update: The V2 stl below fits Lumifel's single print receiver and has mount holes for further modding.
This dampens the sharp gear noise that comes off the Parallax servo, but does not make it overall much quieter, just a bit less grating on my ears. I suggest printing this in PETG, however TPU is probably the best option, I just can't test it myself at the moment. PLA is totally fine, it just won't be as effective as other options.
BOM:
1x M3 14-20mm
1x M3 8-12mm (You can use the one you removed from the servo horns)
Remove the M3 8mm screw/washer from the rear right (or left, depending on version) screw hole on your Parallax Servo.
Run the servo leads through the hole, gently press the cover down as you make sure the servo leads do not get caught/bent.
Screw the 14-20mm M3 into the rear screw hole. If you are using my PTFE receiver, screw the M3x8mm you removed from your servo horn into the front hole.