The blood running through your thighs does the same thing as using a water bottle and it's much less of a hassle!
Simply put the running laptop across your legs and your own body's circulatory system will help dissipate the heat by transferring the heat through your jeans and skin into your blood, which your heart pumps away from the area, cooling as it moves through the rest of your body. The heat is then dissipated by the body through your head, armpits, or scrotum area.
Works great! You should give it a try.