I do like the idea of guides on the side rails. They could be flat stock welded on the sides instead of angle iron.
So why not just cut this assembly off and adjust the angle as you weld it to your trailer. You may have to lengthen both the fixed and folding section to the angle and length you want.
Maybe only the fixed section could have the guide rails. Putting rails on the folding section will interfere with the fold.
Maybe get ScottM or Frank to draw you up some concept plans in SolidWorks to get an idea of who it work.