I have seen CVPI's use these black piece of metals with two holes and a mini-gasket that were installed as part of the "delete option" when the cars were ordered without spotlights but had other police related items installed. Point being, you could use a hand made metal cover with just a bead of RTV silicone or black weatherstrip putty. Priming & painting it may help further if you want to take up this idea.
If this was my vehicle, I could fix it myself by tack welding a filler piece behind the hole that can slip through and stay flat on the backside with a temporary holder that's tacked to the piece. When in place, the temporary piece is removed and bondo is used to start the finishing stages, complete with touch-up paint that matches the vehicle. Second option is a can of expanding foam, shoot a squirt in the hole and trim excess just below the surface to serve as a backer for the bondo.