i just graduated from wytech about 6 months ago. i took the chassis fab class in which we learned how to build all sorts of roll cages and linked suspension systems. it was based around building race cars but same rules apply to all roll cages. i built an exo cage on my toy while i was in that class. i told them i did'nt have the money for DOM or chromoly tubing, and they suggested that i use 1" 5/8" x .120 wall HREW (hot rolled electric weld). basically it's regular welded seam tubing. nothing special. they said that it would be plenty strong if built properly, and they are VERY STRICT on rules and strength of build so i would say that DOM probably is'nt necessary.