We were going to work on building a metal box around the cavities, but they didn't have enough sheet metal and would have to order more, so Frank is working on a design for the box and will order the metal.
Instead, we got free foam! Someone ordered too much, so we took a sheet to build a new box around the cavities to see if it would help with stabilization.
After a while spent cutting the foam, we built a box!
And it is very happy to stabilize the cavities. We placed some foam bits on top to make a makeshift lid, and will use another foam sheet today to cut a real lid that fits on the box.