A light fixture, for a ceiling plenum system defining an upstream space and a downstream space, includes a light-fixture base defining a light-fixture axis. First and second side walls extend toward the downstream space and are sealingly attached to the base plate and to first and second end plates. First and second legs are sealingly attached to the light-fixture base by one of a first-leg continuously welded joint and a first-leg monolithic connection. A downstream end of each leg forms a knife portion. First and second gas filters each comprise an orthogonal gas-filter frame defining an interior perimeter and an exterior perimeter. A trough with sealant affixed therein is sealingly attached to and surrounds a portion of the exterior perimeter. A trough opening faces the downstream space, with the knife portions penetrating surfaces of the sealant, forming a seal between the light-fixture base and each gas filter.