I just purchased some 1/4" 10 PPI reticulated foam from http://www.foammart.com/specialfx9.html - this is the same foam used in the original Maxwell Mural Tanks. I used the Maxwell tank in the late 1960s: it performed well for me.
Thanks for the link. Do you remember the approximate diameter of the tubes? I was thinking of using 3" or 4" PVC, but with 1/4" foam, I think I may have to go even larger for 40"x50"