use some 2.0 DOM tube, its very strong tube and wont dent as easy as HREW tube will. its pricy though, bout 150 bucks for a random cut length of 17 to 26 ft. HREW tube is bout 40 bucks for a 20 ft length. very affordable to buy but not as strong as DOM, but its better then nothing.
or you could go with what alot of guys use which is rectangular tube, say 6 or 8 in by 2 wide, 3/16 or maybe 1/4 max. wall thick. thats pretty heavy duty though. but it makes for a very strong bumper. hope that helps.