There are tons of articles on hot-wire cutters on Hackaday, and I've written up quite a few of them. So I pretty much knew what I wanted:

1. Portable, handheld
2. Capable of cutting 2" thick expanded polystyrene (EPS) boards
3. Maximum depth of cut should be 8"
4. Nice to have: battery powered, adjustable fence for depth of cut - I'll be doing mostly straight cuts.

The first problem was rounding up some nichrome wire. I don't have any in my lab, so I started looking around for some kind of old heating appliance. I was toying with the idea of heading to a thrift shop when my wife found an old hair dryer that nobody likes because it has a rubberized grip that got gummy and tacky. Gross, but perfect.

I cracked that open and harvested all the coils of nichrome. I straightened out one length and got a hank that was like 3 meters long. Way more than enough. I had a spring lying around that OI used to tension the wire, and a couple of cup hooks to secure the whole thing to the arms.

The frame was just found scraps of wood - I literally looked around the shop and settled on the nearest block of scrap 2x4. The arms were from a Lowe's yardstick - thought the measurements would come in handy if I decide to add an adjustable fence to control the depth of the cut.

I had no idea how to power it at first, so I dragged out an old laptop power supply. I think it was 25 volts or so - nichrome glowed bright red in a few seconds. Nope. Tried my bench power supply - 12 volts still too much, 6 volts was too hot, 3 volts seemed OK. So then I tried an 18650 I had lying around - turned the little battery cozy that came with it into a holder by adding on a couple of copper strips for contacts. I added a switch from the old hairdryer to control it.

Works pretty well, although I might go back to the bench supply and goose the voltage up a bit - cuts a bit slow, and I'm sure it'll kill the 18650 pretty quickly. I've got a lot of foam to cut - insulating a cargo trailer. But that's a story for another project...