I would recommend the STC-1000 or STC-1000+.
You *could* wire one up yourself, there are several good threads on it in the DIY forums. But YES, you can start a fire if you hook it up poorly. Odds are you will just pop a breaker or blow a component on the device, but if you are sloppy, stray wires, poor insulation, etc. can create fires under the right conditions.
But if you can understand a drawing, strip wire and screw wires down, you could probably build one for cheap. They are basically a box you get from the hardware store, cut an opening or two, install an outlet or two, wire the STC-1000 and put it together.
The diagrams will vary depending on whether you use a single outlet (break the strap that ties the two plugs together) or install one outlet for heat, and one for cool) and maybe install a bypass outlet just to have a 110V outlet that is not controlled.