Best option........wood blocks in epoxy
Next option.......tape over the hole you have from the outside, drop epoxy in from the inside, when dry, pilot the hole again and just thread the screw in fresh again.

Now, for that cowl. I never just screw through it. I drill out the hole with a 5/32 drill bit, then install a round servo gromet it the hole. Now install a screw (it will have to be a little longer now) with the appropreit size flat washer. This 'shock mounts' the cowl by suspending it in the rubber gromets. Screws will rarely come loose, and the holes in the cowl are also protected from wearing larger over time. Also, the rough inside of the cowl is no longer in direct contact with the covering so it dosen't wear on it. You can touch all this up with paint if you like. I do this sometimes depending on the type plane it is. I mount all my cowls this way and have no problems with screws. Dubro makes silicon gromets for this, but most of us have servo gromets laying around that work better IMO because they are one piece instead of halves like the silicon ones.

As mentioned, be sure to ballance your prop and spinner to minimize vibration.