I've used PTB (they work well), blue foam (I have tons of it now and it does burn). I wanted to get a self feeding mod but decided to try the silicone plug that I read adout here. The cart cover was to thin for me. I just bought a silicone bakeware pan from the Dollar Tree. They are small and come in different shapes and colors. They are flexable but twice as thick as the cart covers. I pressed the atty end of the cart into the silicone to make am imprint and outlined it in ink to see it better, cut outside of the line. I poked a few holes in the middle with a paperclip. Then filled the cart with VG to try in out. The plug fit well and no leaks. Later threaded a strip of PTB in the middle to wick better. It works great for a dollar and I can make tons of them with the left overs.