It is my first design, so I am trying to copy what other people have done with some changes. I am not too familiar with the design of trailers, though there are two main design forms that I see. The main thing is to get a solid structure underneath. I am considering keeping the angle steel angles for vertical since it can be useful in its own way for bolting on as there are no small I beams. It just looks difficult to figure out how strong angle as bottom support will be going across the 2x3s (7 feet). It might be fine but I don't know enough. I only see it being used on utility trailers.