Step 2: Cut dimple mat
The idea for this piece is to space the laptop off of the insulation board, with a ventilated air space underneath. Admittedly, you can use a laptop on a table or a bed without a problem, but I figure more cooling=better.
The dimple mat comes from a company called Cosella Dorken; another company that makes a similar product is System Platon. Unfortunately, you might have a hard time finding it--I have seen Platon at only a few Home Depots. It also comes in big rolls--way too much for the project.
A similar product that you might be able to use is Dri-Cor; I have definitely seen this at a lot of Home Depots--it is 2'x2' squares that you lay down on your basement floor, to create a warmer/dryer floor.