I use tempered MDF, also called Masonite) and 1/4" thick most of the time. For templates that I will want to keep in good condition for a long time I prefer HDPE plastic (cutting board material). I usually buy 1/2 sheet (4 X 4') at a time from a local plastics supplier when they have it in stock, but I have also used about any other plastic sheet that they will sell me if the HDPE isn't available. I don't like to use clear plastic though. I prefer colors, any color over clear, because the clear ones seem to get lost in my shop too easily. Black isn't a very good choice for the same reason.
Central North Carolina