Step 4: Make the tray
Now take the big hardboard/MDF board and cut off an 11 in strip. Then cut the width down to 20 in. You can adjust these measurements for your laptop, this was for a Powerbook G4 Aluminum 17 in. It's not even mine. Remember to leave room for the lip, and you don't want to make the width too small - It's got to fit over your hips.
Cut two more very small strips of hardboard, these will serve as the lip. Make sure they're straight (or straight enough), then line them up on one end of the tray and nail them down. Put a screw on either end of the lip, since nails don't seem to hold very well. Use short screws so they don't poke out the bottom of the tray.