Get the stainless steel locking ring, Item # MREQELRSS from Mancini. While your at it get the rubber gasket too, Item # MREQESUG. This is the only locking ring that I could get to work.
http://www.manciniracing.com/hardware1.html
Also, if all else fails, use Permatex PermaShield on both sides of the rubber gasket.
http://www.permatex.com/products-2/product-categories/gasketing/gasket-sealants/permatex-permashield-fuel-resistant-gasket-dressing-flange-sealant-detail