Building a steel bale wagon

I have a 12 ton running gear that I would like to build a steel frame on for hauling 4x5 round baleage bales. I would like to make a 10 x 24 ft wagon, to get 10 bales on the bottom and 4 or so bales on top. The bales average 1500 lbs. What size material should I get? Should I use I-beams or boxed steel? Any help would be appreciated!

Building a steel bale wagon