ORIGINAL: Vinnie6
I have had a constant problem of the cowl mounting holes getting "keyholed" by vibration. I came up with an idea, tried it, and it works!! I took some fuel line and carefully cut off very thin slices - like 1/16 inch, or one millimeter, to make soft washers out of. I then put Futaba servo mounting flat washers on the cowl screws, with the fuel line under, and turned them in tight enough that the fuel line washers begin to bulge out. Problem solved! The silicone pushing against the cowl keeps it steady and prevents it from vibrating.
An even better variation, is to make the holes just large enough to put the fuel tubing THROUGH the hole.
Then cut some thin tubing slices and put them on the cowl screws themselves under a washer.
Push the tubing through the hole in the cowl and screw in.
The screw will expand the tubing in all directions causing it to grip the cowl from underneith as in your suggestion, but also from the center and sides. This dampens vibrations in all directions.
( This was MinnFlyer's idea and he has a great graphic for it around here... ).