Only the outer face needs to be full width to support the panels and accept the panel fixing screws. These outer faces will also need to be beveled to allow the panels to lie flat. See image alongside. [Not to scale and some angles have been exaggerated for clarity.] The angle of the required bevel is easily found by laying a straight edge horizontally across the ribs.
I was just watching more YT videos on workshop dust extraction. One very experienced chap used a box made from scrap, sheet materials as a collection bin. I would have thought it would collapse under external pressure but it seemed not. One could build a really solid, plywood box for the price I paid for this flimsy plastic barrel. Perhaps this is the way forwards?
Just drawing on the tiny pipe of the DeWalt miter saw was enough to collapse the barrel today. This was on the lowest speed and therefore lowest suction on the Shop-Vac Ultra. I was going to add a batten 'tree' on a length of threaded rod to support the barrel from the inside but the effort seems wasted. I'd have to extract the tree just to empty the dust. A square section box will hold far more dust than a barrel of the same width. Downside is the weight of a sturdy plywood box. Castors would probably be essential.