If using denim - I have noticed that denim comes in different weights - some are almost flimsy and some are almost as heavy (and stilff!) as canvas.
The same with flannel - I used chamois weight flannel (thick, sturdy stuff!) - some is very flimsy.
The ones I made with denim and flannel - I sewed the Xes on the sandwiches - I liked the look better than without the stitching.