I use old newspaper - I try to avoid color pages. I'm always getting some local newspaper or merchant fliers in the mail. With both plastic cups and containers. With a cup, just set it down on the paper and trace a circle around the bottom, cut out and drop in. With containers, you usually need a bigger circle because the holes extend to the sides, so you can trace around the top or just experiment to find a size that works for you. The newspaper eventually degrades and also peels off the bottom of the cup root ball easily when you pot up.