Abstract: In a container assembly for an exhaust emission control catalyst for internal combustion engines, of the kind comprising a cylindrical substrate body carrying the catalyst and mounted within a sheet metal casing with the interposition of a compressible cushioning layer of refractory material, an open-seam sheet metal shroud of variable circumference is used to enclose the compressible layer and to compress it prior to, or simultaneously with, endwise insertion into the casing. The shroud or the casing may have elongated ribs pressed in it, the ribs tapering at their ends to facilitate entry.