Em,
On the two spring comment, a good way to solve that is to use one large diameter spring, say maybe 3/4" or even 5/8", and a second smaller diameter spring of the same length, say 3/8" to 1/2" with the smaller spring inside the larger. They use common points of attachment, and provide the desired safety backup function. A good hardware store will have a wide range of springs, as will a good automotive parts house.