Nice job.
Another method if you want really strong and light is cored composite.
Make your boxes out of 5mm marine ply with the joints just tacked with epoxy. Coat all surfaces with epoxy then fillet the internal corners with epoxy mixed with very fine wood dust (the stuff in your belt sander collector bag works great). Grind the external corners to a radius then lay up inside and out with biaxial glass cloth using epoxy resin NOT polyester.
This method of construction is used to make the boats that nutters row accross the Atlantic in.
You could use a foam core but you would need to use more glass. Ply with glass both sides in very strong. You will struggle to put a hammer through it if you tried.
If you want a bigger radius on the outside, you can build the corner with a timber angle then run a router around the outside before glassing.
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