you mean you are wanting to add a load helper for items that stick out too far??? ten 2x12's at what length???
lets say you have ten 2x12's at 12 feet, in this case not much is hanging out past your bed door (assuming a 8ft bed). that's not much weight at all.
16ft lengths will require some extension help, but again since your bed door is about the center of the boards you wont have much weight out back.
personally, i would go with .25" wall tubing. .125" wall will probably suffice, but .25" gives you the ability to bear more weight and gives you some assurance that its not gonna fold.
for lightweight i would choose aluminum.