The wall is plastic needlepoint canvas (craft store/Wal-Mart). The one I used there is "clear" but it tends to look too white. When I re-do it bigger, I think black canvas will blend in better. I sewed a suction cup on each end with fishing line, because I wanted to be able to detach it from the tank to take it out for rinsing and trimming. This leaves a space between the canvas and the tank wall, but the fish (mine are small types) like to play behind there. You could just lay the canvas against the wall and bury the bottom bit into the gravel, I guess. I wanted something to hide the intake tube of my filter.
Anyhow, I lay the moss on the canvas and made large basting stitches horizontally and vertically with fishing line. It looked pretty unnatural at first until it filled in, the moss was just kinda in checkerboard clumps.