I thought about lots of different materials, and I decided to do PVC because it's cheap and great for ventilation (as opposed to something with a big flat surface.
Because the design is self-explanatory on sight, and because the dimensions would vary with the size of the laptop, I'm not including step-by-step instructions. Also, joining PVC pieces makes precise cuts less helpful because they all cram in a little differently.
A few little notes:
• I used 3/4-inch PVC
• Nobody seems to manufacture 3-way corner PVC pieces that are all 3/4-inch openings. I could only find 3/4 x 3/4 x 1/2, so I bought two adapters. You could also solve this problem by making that lower bar 1/2 but I opted not to.
• Mine isn't glued and the friction in the joints works fine to keep it together and stable. This way I can adjust it if I ever get a laptop that requires different dimensions.
• The corner pieces in front that keep the laptop from sliding down were just a bit short of keeping my laptop secure, so I added some little caps that screw in and add another 1/4 inch or so.
• It would look better spray painted but I opted not to, again to preserve modularity. (It would get messy if I pulled the seams apart and tried to put it back together)