It's easy to make a fireblanket out of an old-generation fire shelter. It'll take a heavy-duty machine to punch through the material though. Been using them for years.
Lay it out, folded in half lengthwise, cut a rectangular shape, as large as possible, though both pieces of material, and stitch up the three free sides (fourth side is folded).
Currently accepted on USFS and BLM-administered rivers, but not acceptable on NPS rivers.