Mine was missing a while back and I ended up using part of a foam gasket from a window air conditioner. I just wedged it in there and you can't tell it wasn't put there from the factory.
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1998 Supercharged Riviera - Custom CAI, Alpine spx-13ref, Infinity 6x9's, Alpine 4 Channel Amp, Kicker KX3, Silverstars, STB, Hawk Brake Pads, Monroe Air Shocks, KYB GR2