Hi there, considering that the catherdral windows were typically done with muslin and you want to do silk; it already isn't proper, as you say. I would think you'd need a fabric that would be strong to support the stitching and the silk maybe a silk blend or you could do muslin or cotton for the background. Your choice of background since it is the main support of the quilt would depend on what you are using it for. If it is for a wall hanging it wouldn't need to be as strong as a quilt would. You could chose one with a design that could be fussy cut so that you could show case the design in the window.