Another thing to bear in mind with using steel wool as the filler, is that even if it withstands the atomiser temperatures and doesn't spontaneously combust, it is still very conductive. Any juice in the cart will get heated a lot quicker as the wool heats up and therefore be more likely to evaporate more quickly.
The ideal would be something preferably less conductive than steel like cotton wool, or some other natural fibre only with a higher melting point... I researched ceramic fibre which can withstand extremely hot temps, but this is apparently carcinogenic when inhaled. 8-o
Silicon is another one and probably similar to what's currently in our ecig carts...