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There are 2 filter designs here, a 3-pole and a 5-pole. Which one you use depends on how selective you want the filter to be, and the insertion loss to be tolerated.
A filter can be constructed for about $10 in parts. A die-cast aluminum box, 2 connectors, aluminum spacers and 6-32 hardware are all that is required.
The filter pictured is a 3-pole, and uses BNC connectors, but SMA connectors have also been used in both designs, and are actually preferred at 23cm.
Click any thumbnail for an enlarged picture.