I use solid body plastics, mostly ... so I don't worry about "filling" my tubes. But, most any kind of thick scent product can be used (Smelly Jelly, Crappie Nibbles, Liquid Minnow, Slab Sauce, Bait Batter, etc). You can also use a small piece of cotton ball, sprayed with a more liquid/spray type of scent, and poke it into the hollow of your tube.
.. cp