Repasting a MacBook

If you have any of the Air models which lack fans, there's a common hack of putting thermal pads between the CPU heatspreader and case, effectively turning the bottom case into a large heatsink, and giving your system a longer maximum performance before throttling.

The downsides is that this makes the bottom of the case quite hot on a place you can touch, but putting a plastic hardshell over the entire laptop deals with that, and also gives protection.

Making the bottom case a heatsink and then putting a plastic insulator around it seems to defeat the purpose of the whole attempt?

Reminds me of the advice given for outdoor electronics: make sure your enclosure is absolutely watertight, and then drill a hole in the bottom to drain any residual buildup from humidity.