I have heard of many people using plastic wads by putting a good non-plastic between them & the powder. "IF" you get a good seal with the over powder wad there should be no problem with melting the plastic.

I have done all my muzzle loader shooting using Card & Fiber, never had a problem with starting fires either. I have heard of people having fire problems by loading with crumpled up Newspaper, but never had any problem with the hard card wads catching fire or even smoldering.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra