What bulb are you using?Everyone wants brighter bulbs, right? The OEM license plate lamps are pretty dim--they don't need to be beacons. The amped-up 194 LED replacements just look wrong.
Back in the day there was paint dip for incandescent bulbs. I'm sure that would work fine on LEDs that run much cooler.
I might try to just spritz some window tint paint on the plastic bulb to see if that works.
I really don't want to do anything to modify the little housings.