you could likely simply tighten the fixture slightly, but
according the parts fiche, its a 16x20 sealing ring, likely you could source something at a local auto parts store, copper or aluminum
( its a metal ring that the bolt goes thru)
10 07 11 9 963 252 GASKET RING - A16X20 (to 12/97) 3 $0.64
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Happy Holidays DrLewall !