I have had good luck making quilting stencils out of DBK Plastic, which is flexible and easy to cut with an Exacto double-blade cutter. The rigid plastic that we use for templates to cut fabric pieces is much harder to cut, IMO. I did try an electric Plaid stencil cutter with a cutting tip that melts plastic, but found it hard to use--it kept getting hung and making big holes in the plastic, instead of a smooth line.
This is a DBK Plastic stencil for marking border quilting lines. Being flexible, it rolls up for storage, which is another advantage:
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Daffy