Hi Dez, welcome back.
I vaguely remember the thread you're thinking of, but it's been a pretty long time. As I recall it was something like a 8' or longer tank, 3' or 4' wide, and wasn't so much an "external" wavebox though as it was a false wall inside the tank that was drilled to accept the Tunze 6200. I believe there were 2 compartments, one each side flanking the overflow on the one side. And he had easily a 6" wave going I think. Is that the one you're thinking of?
Anyhow happy hunting (for that thread).