Thanks! I think I'm going to try what this guy did.
https://www.youtube.com/watch?v=Fk0ahajh71c
My tank is a 20H Tetra, 24x12x16 (I think). I like the idea of having the back and both sides covered, so I'll start with 1 1/2 inch styrofoam and, by the time I'm done with it, it should be smaller.
I may have to Great Stuff some plant holders in, though.