The canvas is there to support the tightening of the belts. The cavase is placed to relieve the stress on the pleats. It you use a bias piece of canvase, it will stretch and the stress will go thru the pleats, loosing the stitches and causing unsightly stitches to show, stretch, and break.


Maybe you could use a piece of leather because it has no bias and will shape to the curves of the back.