If it is a 1/16" or less gap for example, then 3M marine white silicone as a gasket sealer will do. If the gap is larger then use .25" or .50" thick white KingStarboard to make a contoured disc. KS, while impervious to the marine elements, is a soft material and easy to shape. Start with a wide and rounded file and then convert to a radiused sanding block by spray guing sandpaper to various diameter PVC tubes. A mixture of techniques, a few trips to Home Depot, a little elbow grease and plenty of patience.