I was intrigued by the concept of cutting styrofoam so I ordered a Wonder Cutter. It worked great but it was limited in what it could cut because it has a small "throat" (the distance from the heated wire to the frame), so I decided to make a large scale cutter. I used a full 4x8 foot sheet of 1/2 inch particle board with a 2x4 inch frame under it to make a table. I set the table up on two 55 gallon drums, but sawhorses would work just as well. I used a hole saw to drill a 1 inch hole in the center of the table and ran a 2x4 brace under the table next to the hole (If I were making it again, I would probably place the hole 12 - 18 inches from the front instead of in the center. That would make it easier to cut a full sheet in half and you wouldn't have to lean over so far when cutting- hard on the back!). A cut-off eyebolt serves as the bottom hook for the heated wire (Figure 2). Using some scrap angle iron, I welded a bracket to the upright arm and bolted it to the table frame. The bolt holding the arm to the upright is slightly loose so the arm pivots, the spring at the end of arm keeps tension on the heated wire. The arm has a opened eyebolt located over the hole in the table (Figures 4 and 5). I ordered a small roll of NiChrome wire off E-Bay for about $5. Cut an appropriate length of wire and twist a loop on both ends, hook the loops to both eye bolts - the spring will keep the wire tight. I used a Radio Shack transformer (Part # 273-1511B, seems they have discontinued that transformer but it was 12.6 VAC 3A) for a power supply, attached the 2 wires to the top and bottom eyebolts (Figures 3 and 5). You could use a dimmer switch or other rheostat to control the voltage/temperature, but I haven't needed to. I'm no electrical engineer, I tried several things before this power supply worked. I imagine that the resistance and length of NiChrome wire affects how much power is needed.
Total cost of everything was about $50-60. The heated wire cuts through styrofoam like a hot knife through butter, I've tried it on sizes up to 4 inch. The pictures show a wagon wheel I was cutting out of 4 inch foam.
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I don't have a set of plans for this table, I just kind of built it using trial and error, if you decide to build one and have questions feel free to e-mail me