Each piece of foam is the full height of the well required. They are sandwiched together like an oreo cookie. The "0" height wells have a piece of well dividing material as the bottom. The well dividing material I used is 1/8" thuck. There is no backer sheet of plywood (to save weight) but two pieces of straight 5" jamb stock instead.

The top and bottom of the wells are open except where it rests on the support shelf (about 4' wide.) I should mention I went out of my way to build all supports in close proximity to the subs at non right angles (45 degrees angled away) where possible. This in theory reduces the problem of subs humps/coloration (like placing near a corner). I have messed around with a pb12-nsd and a JBL sub beside ikea besta units at my brothers places. Learned some lessons there.