I would recommend that you cut Poret sponges to fit snugly in the available space and cover it with floss. Biological filtration is simply a function of the surface area available for bacterial growth and it's hard to beat a sponge in that regard. Ceramics, rocks, etc. also work, but sponges would likely be the cheapest medium.
But if you cover the sponges with floss to trap the detritus, it's a simple matter to replace the floss whenever it gets too much crap. Wal-Mart sells mattress batting which is the same material as aquarium floss very cheaply.