I placed 9-11x14inch 3mm thick Ambros and 2 14x14inch 4mm thick Ambros in the first completed plate box. Seems OK, heavyish but not crazy heavy. I will need to see how the boxes hang together in the trailer and motorhome while traveling Canada. So far so good.
Will probably make 8 more. Am quite happy to have these now, made from garbage wood in the garage. They are rough but seem functional and relatively strong.