Question/advice for those that have built their own, or have some structural experience.

I have a friend that is a welder, has access to lots of welding gear, and can get whatever materials I need.  He just needs me to tell him what to build.  My question is about how to build the racks to support four canoes, two on top of two.

The base for this is a homemade utility trailer that I just bought and plan to convert to a combo utility-canoe trailer.  All of the framing is 2 inch square steel tubing, all welded.  On top of the base frame are side rails about 30” off the deck, connected to the frame with three posts.  3/16 aluminum decking and side walls.  5 foot tongue that can be extended to 8 feet.  It is built incredibly solid.  The guy who built it is a retired bridge inspector that started his career in welding.

I think the best option for the bottom canoe rack would be 6.5’ long crossbars welded to 6-12” posts that connect to the trailer’s side rails via bolts – so that they are removable.

My question is about the support needed for the upper rack – is a single centered "I" support per crossbar, perhaps with steel plates cut at 45 degree angles from the vertical to horizontal bars for stability – enough structurally?  Or will I need two vertical supports in the middle, separated by 6-12” (or other distance).  If I can get by with a single vertical support, then I won't have to extend the width of the lower crossbars - but I want this thing to be solid.

This will haul primarily Royalex/plastic boats at 60-70 lbs each.