Use the back pocket from an old pair of jeans.
I've been experimenting with making pocket slips from t-shirt material. The stretchiness means you can make it really snug-fitting. When double-layered it feels like it should provide sufficient protection from scratchy keys and coins.
Not that I've tried it but I would think t-shirt material would not hold up very well.
There's a thread somewhere about using shrink tube to make cocoons for SAKs...