Yeah, the guy who made my box put a rubber grommet on there.
The sound I get isn't like a rattling. When the bass hits it just sounds like a dead thump on the sheet metal when you're outside the car. With all the dynamat and the rubber I think I have it isolated as much as could possibly help, but when it comes down to it the box is gonna move when the speaker moves and the box is attached to the sheet metal of the car. I'm thinking about switching to a sealed box just mounted in the back. I'll lose a little space but I would also loose the annoying thump from the outside.