Ok...heres how its done...
go to a craft store and get plastic canvas stuff(white grid looking stuff)
some string, hot glue, needle, cotton quilt batting stuff.
anyhow, measure the inside of your filter where you want the cartridge to go (length/width/height) or just measure the cartridge you are replacing. cut pieces of the plastic canvas to fit each side of this little box your making.
sew them together with the needle and string, sew on the bottom, and sew down one side of the lid to make a lil flap so you can put stuff in it.
put a drop of hot glue on the knots at each corner of the cartridge where you tied off the string. Let it sit to dry for a full two days. or you could just use aquarium sealant. I used glue cause i dont have aquarium sealant.
anyhow, take some flat cotton quilt batting and cut a piece as tall as the cartridge and long enough to wrap all the way around the lil box you made outta the plastic canvas. wrap it around and it should pretty much stick to itself and stay shut.
they fill up the lil box with whatever you want...I put carbon and ammo chips in mine since its on my bettas tank. or just fill it up with bio balls of some sort.
I think it came out pretty darn good!