RE: Covers for servos mounted on wings...
My fix was to put small O ring on the servo arm, be sure its on the arm and doesn't contact the servo case when the arm is instaled. I used a small drop of thin ca to hold the O ring on the arm, Then slip a small piece of heat shrink tubing over the O ring long enough to go from the bottom of the servo arm to the servo case. Put the arm back on the servo and apply a touch of heat from a heat gun to snug things up slightly. Not too much or you will put an extra load on the servo. If done right there's no load on the servo and it's fuel, oil, dirt, gunk proof.
Darren