Turns out I was wrong on the depth of the original amas: they're 7" deep.
The foam material that John has is 2½" thick. I'm planning to stack four up for each ama, to make them 10" deep to start.
Here's where it's at right now:
The foam came in 8' x 42+" sheets. Cutting the edges off, I was able to get boards 10⅛" wide, four from each sheet. The sheets had a tough layer of something like vinyl cloth stuck to both sides that was work to peel off!
The surface of the foam is bumpy and has some air pockets (free, though!), so I'll sand before gluing. John says polyester resin works with it. First, I'll cut all to the template shape...