With BP loads, I did not have trouble with the flame melting the plastic as I put a 2/10" hard card over-powder wad between the cup and the powder. This seemed to protect it just fine.
Seems to me, I experimented with using 2 hard cards and eliminating the lower cup with god results when using the lighter 5 dram load. With a full 6 drams of 2F GOEX, there was room for one card only. I used a folded crimp over the loads in paper or plastic cases. This was in the 12 bore.
It was the 16 bore(gun) that used brass cases and a slight roll crimp. All of these loads used standard 13 bore card and filler wads due to the thin brass.