Hey Ben_M -
You can setup a Soft Spheremask which elimantes the edges of your mesh. I would also really recommend using a particle system here which will not only enhance the heatwaves movement but allow the distortion to always be camera facing.
Here is the Material setup I used in a Heatwave setup, feel free to ignore the Flowmap stuff plugged into the Panner Coordinates, you would not necessarily need that though:
Thank You
Eric Ketchum