There was an article years ago in a Handloader's Digest where the author used a circle of polystyrene cut from meat trays as an ablative "gas check". He found the polystyrene to be just as effective as a regular gas check.

If you simply want a filler, polyester fiberfill used in quilting or pillow manufacturer works well. All you need is a small pinch. Ground polyethylene beads trade marked as Grex by Winchester also work well, but you have to use enough so the bullet contacts the filler so it doesn't mix with the powder. I doubt polystyrene run through a paper shredder would be satisfactory.