10 ga. steel works quite well for boxing in frame rails. Always buy mine from a steel supply house in Sioux Falls. They do a lot of cutting for various projects, and usually have a pile of 10 ga. cutoffs about 8" to 10" in width and lengths of 4' to 5'. Becuase they're cutoffs from a shop work order, I can by them at a huge discout compared to the price of a full sheet, usually about half price.....