In article ,
says...
I'd like to replace my cartridges with plain ol' cotton batting, but of course
my Penguin manual says their cartridges are essential.

You can just rip the old filter stuff off the plastic backing, and
throw it out along with the charcoal. Then you can attach some
"Rite-Size" foam (from the LFS) or batting, cut to fit, with rubber
bands. I notch the plastic backing with a little saw to hold the
rubber bands securely--learned it on the internet somewhere. Works
great and my water is clearer now because of the thicker filter
material. Wheee.