Thanks for your comments.
I used basic sheeting material in cream for the back because it is wide and I didnt have to stitch it together to cover the whole back.
It is fairly heavy and when I was stitching on the border and binding I had to keep rearranging it so the weight didnt pull my machine onto the floor!
I liked using the denim though because it didnt matter if the triangles in the squares were perfect because with denim you can get away with a lot.
I was surprised how much variation of colour you get with different shades of jeans. I was worried when I started that the colours would blend into each other.