I use a coffee can lid. Cant remember what size, 3# maybe? it fits perfectly on most of the chapparral feeds on the C band horns. Never had a problem after that. They will need replaced after a couple years, but the price is right. Michael Heim Chief Engineer, Forever Broadcasting New Castle PA WKST WJST WWGY 814-671-0666 Chapter Chair, SBE-122 ARS KD0AR