I used cardboard cut from cracker boxes. I like the color that they give for the substrate.
I'm not happy with some of my gluing on these. The paper is silk screened color and very sensitive to anything on it. The adhesive moved around a bit when I burnished the pieces down. Oh well.
You can see what I accomplished during my 365 project here. And you can see what I've done since that project ended a year later here.