You might be able to triangulate the rear span with bolt or clamp on tubes going from approximately where the upper rod holder is on the uprights to where the outer rod holders are on the top crossbar. That won't be quite as good as a weld, but maybe easier to try.
Watch the 3/4 ply pads. They will slowly rot. That is what wood does. I would prefer solid glass there. Tapered out gradually in thickness to reduce the stress raiser and chance of a crack. It isn't a lot of weight and will never rot. Second to that would be a high compressive strength cored composite. Like balsa. I love foam but it probably isn't correct in that location. But it hardly seems worth the effort for a small area like a pad. That's just me. I'd do solid glass with a lot of taper.
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