NOVELTY - A gas treatment device has a secondary support made of an inert heat-resistant material and located in at least a portion of a substrate; and a primary mat support made of a fibrous heat-resistant material and located at the substrate. The substrate and the supports form a subassembly located in a housing. The primary support has a region adjacent the secondary support. A selected mount density is produced in at least a portion of the region.    USE - Used for gasoline and diesel engines for vehicle applications, non-thermal plasma reactor, adsorber for hydrocarbons and oxides of nitrogen, and fuel cell reformer.    ADVANTAGE - The device mitigates the problems caused by variation in the size and basis weight of individual components in a gas treatment device. It solves the problem of being unable to feasibly specify as single optimal basis weight for the mat support of each individual gas treatment device in a production run.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (A) a gas treatment system, comprising the above gas treatment device, and an exhaust system component in fluid communication with the housing; and    (B) the production of a gas treatment device, comprising determining a first selected length for a primary mat support (4) for a substrate (6) of the gas treatment device, where the first selected length is sufficient for the primary mat support to be disposed completely about the substrate concentrically; determining a parameter of at least one subcomponent of the gas treatment device; determining a second selected length for a secondary mat support (2) for the substrate based on the parameter, where the second selected length is sufficient to produce a selected mount density within the gas treatment device; forming a subassembly by disposing the first and second selected lengths concentrically about the substrate; disposing the subassembly substantially concentrically in a housing; and producing a selected mount density within at least a portion of a region of the primary mat support that is adjacent the secondary mat support.    DESCRIPTION OF DRAWING(S) - The figure is an isometric view of a subassembly.    Secondary mat support (2)    Primary mat support (4)    Substrate (6)    Secondary mat support edges (12a, 12b)