Here's what I used for the longerons which run on top of the stringers (36" C to C). I used 1" square tube and fastened it to the stringers by welding 1" wide steel straps on each side with a bolt through the straps and stringer in three locations from bow to stern. All the superstructure was welded to the square stock. Not a great pic but you can faintly see the strap by the back of the hammer in the pic.
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