I bought six new Grand General 7" LED sealed lights for the back of my bus. My old lights were all leaking badly due to failed seals. The problem I'm facing is how to install these so that they will be watertight. The original lights consisted of the plastic face attached to a metal plate, with a gasket between this plate and the bus skin; the lights were attached with sheet metal screws into the skin.
These new lights do not have a metal backing plate (they're all plastic plus the gasket). The skin on my bus where these lights go is now slightly warped and floppy (from some rust on the inside), so to stiffen the skin and ensure a good contact with the gasket I'm putting a piece of 1/8" galvanized steel across the holes on the inside (I'll be riveting this plate to the skin). I'm then going to put tapped holes in this plate for attaching the lights.
My problem is that the four screw holes on the lights are outside of the gasket, so I think those holes may leak a little bit. I could use seam sealer on the holes, but then I would not be able to easily remove the lights if necessary. Maybe I should use rivnuts here instead of tapped holes?
What's the best way to install lights like this in a watertight manner?