I kicked this around with some mechanical engineers at work. Visio uses solid bottom construction, the side panes sit on the bottom pane. The theory is the foam will fill the space under the bottom pane as the trim compresses the outer edge. This would help spread stresses as the bottom pane bows. It seems this would become more important on bigger tanks with giant bottom panes of glass. (sketch not to scale :clown
Good luck, I love tools!