Fabric
Medium weight, dress weight wovens, cotton sanded twill, 8-14oz denim, linen, corduroy, tencel, tencel/linen, wool tweeds and oilskin. It works well in fabrics as diverse as heavy wool and oilskin. If you want to keep the boxy shape choose something with structure and body, if you use something like linen it will have a softer more slouchy look.