Tool - Craft Knife
OLFA 5019 SVR-2 9mm Stainless Steel Auto-Lock Utility Knife
https://smile.amazon.com/gp/product/B0006O87O6/ref=oh_aui_detailpage_o02_s01?ie=UTF8&psc=1 $9.93
Spare blades
https://smile.amazon.com/gp/product/B0006SJAPW/ref=oh_aui_detailpage_o02_s01?ie=UTF8&psc=1 $6.83 for 10
The reason I use a small (9mm) blade instead of something larger is that you get reduced drag when cutting polystyrene.