I bet if you were to buy a large "FeatherRock" from your local garden supply store, this could be a really effective bio-filter medium. If you don't mind taking some time to either break up (hammer, chisel, etc.) or grind to the desired shape (Dremmel, grinder, or hand file), you could make this fit into any space you like. This stuff is soooo porous too! Much better than lava rock for bacterial colonies. It's also extremely light (IT FLOATS) and wouldn't weigh down whatever you put it in.
Personally, I'd Dremmel it into roughly spherical shapes about the size of a marble, and place them in the filter of choice. This stuff is easily broken, drilled, or ground down.
Other simple items include Army Men (many people use these), or plastic easter grass (the green plastic strips wadded up). Just stuff it into the media area.