Wow Ron, You have quite the collection of 342 pics. I recognize some of those, but not all.
I found a stock steel channel that was almost the right size. Had to grind quite a bit to get to fit. They are difficult to install and remove. I made a drawing of a formed channel from sheet steel. It would be a drop in fit, and would still allow for the use of the bows and canvas, like the troop seats. I had a local sheet metal shop quote me about $20 each. I told him to go ahead, but we both forgot about it.
I was using a treaded 2x12 I think. I recently had a neighbor give me some freshly "bandsawed" oak. (4 pieces rough cut 1 1/2 x 8 ) It is drying right now and hope to install someday.
I am torn because I want to be able to use the bows and cover, but that requires mounting the sideboards inward. (looks goofy imho) . I rather like the pics above that show the sideboards mounted directly above the stake pockets.