Has anyone tried a stitch and glue boat using 2mm plywood? Even if used only for the sides, it would lower the weight. I would not necessarily expect it to be "rugged", but compared to sheet aluminum, I would trust it a LOT more. Even light weight fiberglass inside and out would further strengthen it. I would use only top grade mahogany plywood, and not other stuff with "acceptable flaws".
A "light duty" boat has to be used for just that - light duty. Here in the Great Lakes area, there are a lot of rocks, many of them are igneous, from volcanoes. They all come fully equipped with sharp edges, and sharper corners. A few have mastered the ability to leap into the air, and position themselves right where I intend to land. They can also become invisible while kn flight, and penetrate water without making either ripples or a splash! They're sneaky, and bear watching. A light duty boat would require a very careful paddler here. I'd best stick with rugged.