DogFish has some good advice up there.
I just want to second the bit about carbon not being necessary. The two main uses I can think of are trying to remove either medication or tannins from the water (which I would suggest not doing, they can be pretty beneficial to both plants and fish).
Back when I started, I had one of the old Tetra HOB, and it had a similar cartridge system. This was back when I still thought carbon was necessary.
I'm not sure specifically what I did anymore, but I cut open one of the old cartridges and either reclosed it, or encased it in nylon or a media bag or something similar, and would fill that with carbon that I bought in bulk containers.
If you have trouble fitting a sponge, you might be able to strip one of the old cartridges and use the frame from it as something to embed the sponge inside of.