Early on, when I was still using E-Cig products, I bought aquarium filter material from a pet store. It's polyester (not good to inhale if it burns or vaporizes!). I found that material to be identical to E-Cig's cartridge filters.
As an experiment, I set fire to an E-Cig cartridge core and a compressed wad of polyester material side-by-side. Same melted lumps resulted. So I packed cartridges for a short time (proper compression is a problem). Then .. dropped it. Safety unknown. Dropped E-Cig. Safety really unknown.
I've concluced Janty carts aren't so expensive that I can't afford them. The longer I do this, the less happy I am with fiddling around with "make-do" stuff. I never rolled my own cigarettes (after trying it for awhile), so I'm not prone to "roll my own" e-cig cartridge cores. Just personal preference though.