steel
Dave,
i used 1.75 square tube, 1/4" wall
I used to work in a fabricating shop and so tend to overdo things, note the gussets at the joints. Those dimensions are actually okay for structural welding but you never know when you're going to want to put something heavy on like a load of lumber, so I beefed them up. I welded AND bolted the tongue to the frame, its 2.5x2.5, 1/4 wall 14' long. We wasted about 20 mins trying to square the tongue to the frame, then gave up and squared it to the axle. Turns out the frame was off 3/4" on a corner to corner measure, thanks Taiwan
if your grill guard is round, try muffler clamps so you can remove it entirely for a cleaner mount, or axle to spring clamps if its squared and "t" the bottom of the upright. Then you dont have to worry about the weld if the guard is thin walled.
If I find I want to extend the bar width, I'll likely bolt a longer piece of oak to the crosspieces for extensions that are easy to remove, since a heavy canoe is 75 or 80 lbs thats no problem, and I dont often move more than one at a time.
'nother shot of the front upright, easily adjustable for length.
By the way, the same stuff in aluminum was so $$ when my heart started again I had forgotten the price the guy quoted me.
hope this helps.