I think the ideal material would be fiberglass, but I doubt anybody would want to use it. Think ZFM, but instead of a piece of silicone with a hole in it, you would have a small mat of woven fiberglass. Wicking should be very good, and glass certainly won't burn up. You'd have to fill it with a syringe, and I'm not sure how you'd attach it to the cart reservoir, but it would solve most of the other problems with cart filler.