This is as true for insulation board as it is for the ubiquitous ... is a simple build based on a single 18650 and a piece of nichrome wire. While this is probably not the most Earth-shattering ...

If you don’t already know it, nichrome wire is nifty stuff that’s readily available in thrift store hair dryers and toasters. It stretches as it heats up, and shrinks as it cools back down.