An insert adapted to connect to opposite walls in a heat exchanger tube, including a corrugated sheet having alternating wave crests and wave troughs connected by wave flanks having openings therein, wherein at least some of the wave crests have a length different than the length of the wave troughs, and/or adjacent sections have different wavelength waves. Such inserts may be produced by transporting material sheets through a press, where the sheet feed rate and/or the press stroke speed may be selectively varied.