If anything you want to put a hard, flat surface on your lap to place your laptop on. Like a wooden board or tray. This lets the feet on the bottom of the laptop do their job and keep the air circulation open.
The idea that using a laptop on your lap will make you sterile is wrong. While I suppose that if you set a hot laptop on your lap for many hours, and then ran a test on your ejaculate, you may end up with a lower sperm count for that one emission, I don't imagine there would be any damage to your testicles, only the sperm produce in that time frame. Also, all the laptops I've used have the heat vents near the back of the laptop, not the front. The heat would be coming down the middle of your thigh, not your crotch.
While I'm no expert, I'd imagine that wearing your tighty-whiteys for your whole life does more damage than a few hours of laptop-lap heat exposure. Those keep your testicles up against your body and unable to naturally lower to cool down.