Take some brown leather (bought from a craft shop or cut out of a thrifted mens leather jacket) and a thrifted trench coat. Cut the leather into shape by tracing the outline of the back weather panel of the tench coat. Glue the top edges of the leather to the trench coat as seen below, so that you leave the bottom of the panel hanging. Glue the same leather to one side of the trench coat's belt.
Image: Anywho