polyethlyene or Polyproplyene or flexible PVC maybe. i once bought some really good white flexible plasic sheet 3mm thick from Bunnings for panneling on a sidecar and it was really good stuff and impossible to break. You can bend it in half and it will not break at all. Just look through yellow pages and ring some places and tell them you need something 3mm thick or what ever and it has to be flexible. I did that the the otherday when looking for some black sheet for a sidecar i just built. The stuff comming is black 3mm thick polyethlyene.