Sure, I made the foam cutter using 18 AWG nichrome wire (sourced from my local electronics hobby shop
Norvac) and a (hastily) home made wood tensioning device. I used some left over surgical tubing to keep it tight as you can see on the top. That could easily be a bungie or some other stretchy material. I then attached a regular power cord with one wire going to each end of the wire and used some bolts to clamp all of that together. DO NOT PLUG THIS DIRECTLY INTO THE WALL. I used a variac transformer and started from 0 and slowly worked the winding ratio up until the wire started to cut the foam cleanly. You can also use this
calculator and figure out exactly what voltage you need and create it some other way.
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I then made a jig that held the uncut foam. On the edges I put rails that the nichrome wire could run on to make the appropriate arc cut. I used 1/4"x1" aluminum bar stock, you need to be very careful using a conductive material as the voltage between the two bars is close to the output of your transformer and can hurt/kill you. I didn't have anything flexible, nonconductive and heat resistant, so I made the aluminum work, but we were very careful with it.
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Then it was just a matter setting up the foam in the jig and dragging the hot wire cutter through it. Run a few tests on some scrap foam first and be in a well ventilated area! Be careful to keep a consistent speed and even progression from each side. Stopping is also a big no-no as it burns the foam.
This was my first time doing it this way, so I am by no means an expert. Youtube "nichrome hot wire foam cutter" and you will find lots of other (better) examples! Good luck with your hot tub cover.