Having made my share of saddles over the years, I've discovered denim or canvas will hold its shape and remain in place better than thinner fabrics. I've taken advantage of colorful scraps of a variety of fabric by doubling or tripling the layers and sewing them together.
Hens are Houdinis at getting out of these garments. I've found that velcro at the end of half to 3/4 inch elastic works the best, and if you can bring it up under the wing pits from the side of the saddle and fasten at the top of the shoulder toward the back, the hen is least likely to be able to pick loose the strap and velcro.