Ive kicked a similiar idea around however instead of a matrix box I Iwas gona use a heavy duty grate system that we use for our pondless waterfalls. They are only roughly 3" thick and hold more weight than the boxes. Also it cuts down on the additional excavating needed to build this design. The one major difference tad in my set-up is that I am incorporating a bottom drain/air diffuser combo. This combo will help keep the bottom from "clogging" and provided additional aeration. I love this idea because of the dual purposes it creates and also helps sell your ponds even more.