Yeah, clips all the way!!! I used to use heavy books as an extra precaution too. But my friend said that she did that for one of her boas. The boa managed to lift the lid and books just enough to get her head out but couldn't maintain that enough to get the rest of her body out or her head back in. She ended up decapitated. After I heard that, I ran right out and got a bunch more clips. Probably not a real common occurence, but I'd rather not chance it.