Aquarium corner filter replacement?

I have an economy corner filter a href="http://www.bizrate.com/fishsupplies/oid355708192.html" rel="nofollow"http://www.bizrate.com/fishsupplies/oid3…/a and I was wondering what is the best stuffing for it? It came with this nylon-like floss and a little bit of charcoal. But I'm worried that when it comes time to change the filter it'll be expensive and/or inefficient. Is it possible to just cut snug-fitting triangles out of a sponge and stack them in the filter?