be as accurate as possible. If the openings have been taken out on the upper at all, you'll need to match them to make sure that the air isn't running into a small air wall on the edges. If you plan on doing this yourself, I would suggest closely measure each individual one and compare the same chamber to it's upper counter part. If it's off by a large portion the air will hit a bump when it get to the lower plenum and screw with the turbulance of the air, which is the opposite of what you want.... better airflow. Hope some of this helps. Here are some pics of some stuff I did recently.
