This is what I did because I don't have a lot of tools, I used a craft knife. You can light a candle to heat the blade in every few minutes because that will help melt through the plastic as well as cut it. First make shallow scores so that you know where to cut and then cut deeper one section at a time (a few cm at a time) and go round the whole thing. Once the cut is deep (almost completely severed) sink the knife all the way through at a point and then slide it along the cut slowly (moving the blade up and down as you go along). That should do it!
