I'm putting some rows of these diffusors up on my side walls, near the ceiling. I currently have 16, but now realize I'm going to need more, so I'll be ordering some more in the coming weeks.
They are hollow on the inside, so I basically just wad up a big chunk of polyfill for each piece, and nail them to the wall. Its a bit tricky without a second set of hands though, as I came to find out today.
I think I'm going to put one row on the walls, and another basically right up against them on the ceiling, which, because of their proximity to each other, should sort of act as bass traps in addition to diffusion/absorption. Well, not bass traps per se, but they will help de-square the corners of the room a bit. They should do a little bit of everything, really.