If you're planning an "H" style press your 4" channel should be plenty, DEPENDING on how thick the material is, and what design you use. Coal Iron 12 ton press uses 2"X1/4" angle for the sides (2 on each side) with a 1"X3" solid bar set edge ways for the ram supports. There are plenty of examples around for you to look at. "IF" you're staying below 12 ton "H" style the Coal Iron is an easy design to follow.