I've ordered the RF Gasket today from East Coast Shielding based in New Jersey. They were real easy to work with and reasonable in their prices. The cost of the gasket was at $100 plus shipping, however this will eliminate RFI and EMI leaking through the top of the can.
What I ordered was a product called Wire Oriented Silicone, this will also keep the weather seal of the can as well. That was a requirement of the gasket in addition to the shielding properties.