Though I haven't tried them I've seen posts using every type of material from metal mesh to the replaceable filter material used in fish tanks.
Some cartos use a single coil at the bottom that creates a hot spot, which can burn the filler material easily. Others use a dual coil to distribute the heat more evenly. Still others make a horizontal coil, which is basically a wire that runs the length of the carto. This spreads out the heat more evenly yet. A lot of the Boge's use this design.
I've torn apart a bunch of cartos to see what makes them tick. The different designs are interesting to see but it's always easy to spot one design flaw or another. Since I'm a supplier I'm not really allowed to push one product over another on this forum. I will say it pays to experiment with different products.
My personal opinion is that the filler's main job is to keep liquid from leaking out of the top. That way you can carry it around, lay it flat or even turn it upside down and it won't leak. The problem, then, becomes one of the filler coming in contact with the coil and eventually burning. That's why a lot of mods out there don't use a filler at all.