|Above left is my design with a twisted rope tensioning device.||On the right is Martin Slater's adaptation. Martin uses a tension spring and a chain for tension. The tension is adjusted by chain link and it is quickly applied or removed for threading the wire through holes.|
The design is simple and you should be able to make it from scraps that are lying around.
You can alter the dimensions to suit what you have or make it bigger or smaller.
Right click here to download the dimensioned drawing in pdf format.
The easiest power supply to make is with a battery charger and a lamp dimmer. Wire the lamp dimmer to feed a duplex outlet in a 4" steel outlet box with the correct cover and a grounded cord. Then just plug the battery charger in and control the current using the lamp dimmer.
Start with the dimmer turned all the way down and slowly increase the power until you can melt the foam. Too much power will vaporize the wire sending little red-hot balls of metal all over the place so don't use it near anything flamable. The dimmer/outlet arangement can also be used to control soldering irons and other resistance loads.
WireI like nickle-chrome (ni-chrome) wire, but you can use stainless steel wire and even guitar string. You need a single strand between .010" and .030" diameter. The thinner wire will stretch straighter and require less power to heat. The thicker wire will require more power, but is less likely to burn out. To make a stiff wire straight, put a loop in each end and with a nail through each loop, twist while pulling straight. Ni-Chrome wire is not un-common and it can be purchased from McMaster-Carr.