Thats exactly what I do, its not conductive, so I just have the PCB horizontal and put them on top of it and immediately you can see the hot spots, if there are any. Its very sensitive.
They are just plastic, the best ones I have are square and approximately 15x15 cm. Thats big enough for most PCBs I need to do it on. You can move it while you're using it, the color changes in real time, almost instantly.
I have accumulated a bunch of them from different sources. The ones I use the most are sensitive between 20-25 degrees C