I used some expanded foam packing (not styrofoam). I got it from our IT dept, it's what HP uses to ship their computers. I thought I had pictures of it, but I think my better half has them. I'll post it up later today. Your layout looks just about like mine. I used Roxul and 1" foam board and Reflectix stapled to the floor. Here's a picture of my layout minus the packing on the side of the tanks
Anyway, it's pretty dense and it fits perfect in the channel of the frame.