Threw together a hot wire cutter last night with a bit of cut 5mm MDF a couple of nails, the ni-chrome wire and an old Sony 4.5V/500ma AC adaptor that I seem to own for some reason. I can’t find the device that relates to it so I sacrificed it to the cause.
The AC adaptor is powerful enough to warm up the wire enough to cut polystyrene with a bit of force applied. However I need to grab a 9-12V/1000ma AC adaptor from Dick Smith to really heat up the wire so I can just slide it through polysytrene easily.