I use a lemon for this purpose. Just thread the lemon onto the rotisserie rod where the chicken will be. Make sure that you have one fork on the rod, on the handle side. Place it pointing towards the middle on the handle end of the rod. The lemon will fill up the cavity of the chicken and help hold it in place. As the chicken cooks the it will shrink a little giving it a better hold. If you don't like the lemon flavor on your chicken, you can use something else. Try an apple, onion, or even a potato. The potato will be harder to get onto the rod, but the neutral flavor won't affect the taste of the chicken as much (if that is what you are after). Just try to pick something that will fill the cavity sufficiently.