Sticking a hot laptop into a sleeve and then into your backpack is kind of like you going on a 50-mile ride then wearing a down coat in a sauna.
The Timbuk2 Crater Sleeve protects your laptop from its own internal heat as well as external bumps and scratches. Made of a high density foam with lots of tiny holes (a.k.a. craters), it allows your laptop to release heat, and cool down more quickly. (Laptops like that.)
Plus, like a good laptop sleeve, it protects your laptop from scratches. An L-shaped zipper lets you access your computer from a vertical or horizontal orientation.
Comes in sizes for 11-inch laptops (such as a MacBook Air) up to a 17-inch laptop (such as a MacBook Pro).