I've had success making my own stencils with 4ml painters plastic.
There is even heavier weight plastic you can choose to use.
They're lightweight, easy to clean & store. If you need them slightly stiffer, tape edges with heavy duty duck tape.
Nice too since they're transparent, you can place over design and trace.
If you want to repeat your stencil designs, they're thin enough to use as a mask for silkscreens.