Sometimes I finely chop some veggies inmy food processor...yam, carrot, onion , red pepper, zucchini and add it in with a couple of eggs to bind it. I make these in muffin tins and put them in my kids' lunches. Eaten cold.
But you don't need 'filler'. Just meat and spices is good. Google 'paleo meatballs' for some recipes.