That's good; I've got some scraps of WM pad foam I can use. I might sand it down to the preferred thickness (with a mask and goggles of course).
Also, where do you source the metal for your hoops? I'm going to try without one first, but I may need to add one afterwards if it's not enough support.
My idea for getting enough support without a stay is to roll an inflatable pad around the inside of the pack, load all the other stuff into the middle, and then partially inflate the pad until everything's nice and rigid.