I'd like simplicity, but it has to work within the constraints I have.
I'm also suffering from a lack of information.

Spancrete panels with spray-on foam on the underside sound like just
the thing. Floating isn't relevant, since it has to stay up when I
drain the cistern for maintenance. But strength in compression (the
good thing about blueboard) is irrelevant for the top, and I agree that
spray-on foam sounds like it would stick just fine. I'll look into
these.

Thank you.