A catalytic unit, a process for providing a support mat for the catalytic unit, and a process for assembling the catalytic unit are provided. An installed mat density for the support mat being calculated based upon a desired annular cross-sectional area of a gap between a catalyst carrier and a shell of the catalytic unit, with the support mat being sandwiched therebetween. The support mat for the catalytic unit can be provided by first slitting a bulk roll of support mat to form a plurality of end unit specific mat rolls. The support mat can be wrapped around the catalytic carrier to form multiple layers of support mat, with the support mat having beveled leading and trailing edges to reduce variation in material density in the layers of support mat overlying and underlying the leading trailing edges. The support mat can be free of any binder.