obviously its not going to be robust as the metal disc but good for custom shapes or one that you will only want to use once or twice.
i used 'shrinkles' brand shrink plastic (this stuff: www.wizardtoys.com/
i drew a circle 3.9cm in diameter (my gun is a cheap and basic with discs that are 1.5cm in diameter) with a simple scale shape in the center and cut it out.
(other brands may shrink by a differant % or if your gun is larger you may have to experiment to find the size you need)
bake according to the manufacturs instrcutions.
you can sand shrink plastic too so if it dosnt fit exactly or if your shape is a little rough grap some wet/dry sand paper (i used 400 grit) and sand away!
the shrinkles disc is thicker than the metal discs but still fits securely.
however shrink plastic dosnt react well with unbaked polymer clay even after 24 hours it started to go stringy and change consistancy (see photo. bottom right) so you will want to clean the disc thoroughly with wet wipes etc to remove any clay after each use.