In my design notes, the rounded corner lip was supposed to be plywood. Having done so much in metal, I just can't bring myself to fall back on wood. Blah. [sticks nose in air]
So I traced out the corner lip in some scrap.
Clamp, drill, disassemble, deburr, construction adhesive, rivet.
Also need a heavy bead along the inside to make sure this is sealed up.