What's a good way to plug up a firewall hole?

[In The Ten Ring]

Now that I've decided to put a newer model windshield washer pump under the hood, there isn't much need for that 3-hole grommet in the firewall. Is there a solid grommet I can shove in there? What else can I do that doesn't involve welding, rivets, or bondo? (And duct tape, let's not forget that solution!)