Worse comes to worse, aluminum foil. Even thats not great for you...but, a hell of lot better than takin' a lung full of plastic.
whatnow? a mold? nah. just find something else to toke out of. search for homemade bongs bro it's not very hard to find decent materials to use.
probably a home-made bong, but if you bought a bong with a plastic stem the fuckin return it. DO NOT smoke plastic no matter what you put in it, unless you dont mind smoking highly toxic highly dangerous fumes.
it is fine to use plastic as a stem just not the bowl. I use bic pens for quick on the go bongs all the time I am still alive. as long as the flame does not burn the plasric ur good.