A 'hot wire' is a wire, nichrome (see the local airport's FBO for some 'safety wire') or music wire (like a guitar string) that is strung to a bow - like an archery bow string. Apply electricity to both ends (a HO train power supply works good), and it gets hot enough to melt its way through the foam. You make templates for it to ride on, tack those to each end of a block of foam, and slice it out...