Thanks Cramden, I probably shouldn't mess with the "recipe" too much. I.e. - a thinner plywood might cause problems with the screws coming through. And no plywood at all might cause problems when I try and create a river or pond by carving into the foam.
I am thinking that 1/2" birch and 1" foam might be the combination that I am looking for. Thanks for the hint on the "top attachment blocks" that I found at the Mianne site. I am getting the Mianne used, so might need to order odds and ends. Appreciate the pics.