A soft rubber gasket on the bottem side (inside the trunk) would be your best bet for water tightness. You can pick up a set for cheap at any hardware store. Just make sure and get the ones designed for plumbing. As they will be water tight when tightened properly.
I still think it's important to put a fine layer of (preferably marine grade, for boats) RTV Silicone adhesive on the plug before inserting it into the hole and quickly wiping any excess off of your paint with an alcohol soaked cloth. The rubber gasket underneath would be a great plus, but I say to use the silicone because I feel that if you only use the gasket underneath, water, road salt residue, etc. could be trapped between the top of the plug and the gasket, leading to accelerated corrosion.
I would also suggest buying 2-3x as many plugs as you need to do the job once. When you remove them in the future to put the wing on, you will likely damage the plug while punching it out from the underside. Also, periodically replacing them will keep them in better shape. Sun damage and seasonal temperature swings will make them brittle and compromise their sealing integrity.
Honestly, I kinda like the machine screw and nylon washer idea too!