An exhaust treatment device comprises a housing defining an inlet opening and an outlet opening, a first catalyst brick and a second catalyst brick each having an inlet end and an outlet end, a first insulating support cover, and a second insulating support cover. The first catalyst brick is disposed within a first segment of the housing, and the second catalyst brick is disposed within a second segment of the housing. The first segment has an inner periphery that is not equal to an inner periphery of the second segment. The first and second catalyst bricks each have nonuniform dimensions with respect to one another. The first and second segments of the housing are independently dimensioned in proportion to the first and second catalyst bricks respectively. The first insulating support cover is disposed within the first segment of the housing in a first annular space between an inner surface of the housing and an exterior surface of the first catalyst brick. The second insulating support cover is disposed within the second segment of the housing in a second annular space between the inner surface of the housing and an exterior surface of the second catalyst brick. The first and second insulating support covers are independently dimensioned in proportion to the first and second catalyst bricks respectively.