According to one embodiment, an apparatus (50) for mounting to an inner wall (33) of an exhaust tube (32) includes a tube engagement surface (52) and an exhaust engagement surface (54A). The tube engagement surface comprises a convex surface of a constant first radius of curvature about a first axis. The exhaust engagement surface is adjacent the tube engagement surface, and includes a concave surface of a second radius of curvature about a second axis generally perpendicular to the first axis.