Buy some O-rings that have an I.D. just slightly smaller than the O.D of the skid plate screws. On my bike, the skid plate screws are M6's (6mm) and I found that an AS568A-009 O-ring (7/32" I.D. x 11/32" O.D.) works perfect. You can buy a bag of 100 from McMaster-Carr for less than 5 bucks. I like the EPDM material because it will not crack from ozone over time. You can order them online at www.McMaster.com (P/N 9557K461).
One other tip - countersink the ends of the standoffs to create some space for the O-ring to go when you tighten the screws. The standoffs are typically plastic or aluminum so you can do this easily with a hand drill, just use a bit that is as big as the O.D. of the O-ring and go about 1/16" to 1/8" deep.
Happy wrenching
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