I’ve just noticed Sally 4th sell a polystyrene cutting template for £3.50. Think I’ll add one to the small order I’ll be placing. I can’t cut a straight line to save my life. Last night I tried to cut a line between impressions on a new silicone mould just so it would fit in a storage box. It did not end well. Now I need to make a trip to MB Fibreglass for more silicone and make a mould box inside a mould box.
Chris Abbey also has a couple of videos and an instruction PDF on Terra-formers. Best of luck with your cutting. I watch with interest.