Don't use one.
One laptop I just put on a couple of place mats for plates so that it doesn't mark the table. The other has an extended battery which raises the back of the laptop so most of the bottom doesn't touch the table, or your legs.
If heats a big problem, then maybe theres a problem with the laptop, you could use power saving to run it cooler. Maybe you need a different laptop, one thats just runs cooler.
If non of the above is possible, then perhaps one of those cooling pads might help. But its seems a gimick that isn't really solving the problem. If you remove or reduce the heat, you don't need the pad.