When I cut plastic of 5mm and less I use masking tape both sides of the
sheet to be cut for about 2" either side of the cut line. That gives me a
surface I can put a pencil mark on and prevents chipping at the cut edge and
any scratches from the sole plate of a jig saw which is used at slow speed
with the correct blade for plastics. I also clamp a piece of wood either
side of the sheet with enough depth on the underneath piece to keep the saw
blade clear of the bench. If you position the top piece correctly it can act
as a straight edge for running the edge of the saw sole against. (assuming
of course that you are wanting to cut a straight edge)
FWIW Peter...