Not boxing for strength, more to do with rot repair in my siderails, but along the same line. Here's some pictures anyway of what I'm doing with my frame right now. The "caps" are about 14ga, are made from Galvannealed HSLA high strength steel (I work on a Galv line). This hard material was not easy to bend in a smaller brake!
Like Mr Bruce said, make a template duplicating the frames cross section and start forming.
Good luck!