cardboard will distort if used often.
some will use old x ray film if available.
Any plastic will work. Just make sure it is flat.
Plastic lids, etc etc.
It is useful to glue a strip of fine sand paper to the back to keep from slipping. If I am making one a few shapes, I will cut out of graft paper and then use rotary cutter to cut using the paper as the shape. Have to be careful for accuracy.