Hi Chris,
I've been doing this for years and never had one come loose. Inside the cowl where the screw holes are ,epoxy a piece of 1/32" plywood and cover it up with a piece of light glass cloth. Fill the existing screw holes with epoxy. Drill and tap for 6X32 Nylon bolts and use them as needed. The bolts will stretch and not come loose. Since they are plastic they will not wear out the cowl and the wood is tougher than the glass is to vibration. I've use this for all sizes of planes from .25's to 120's and latest a Moki 210 on a Sundowner. The larger planes might require 8X32. Good luck.
Tom