FWIW, the clear plastic clamshell boxes that fruit and lettuce comes in works like shrinky-dinks plastic. You can cut out pieces, color them with Sharpies, place on aluminum foil and bake in a toaster oven.
Daiso also sells shrinky dink sheets (non branded, but the same stuff).
Single sheet per package, about 10" by 14".
The packaging changes, but this one is similar to what we recently purchased:
would HDPE (same stuff that milk cartons, many condiment bottles, detergent bottles, and a lot of other household containers are made of) work as well? Can that be properly sterilized? Google has been inconclusive