Well airflow on internal components isn't an issue, since you can water cool just about anything. If you are really set on doing this, you should have one intake port in the desk with a filtered fan blowing in. Then you don't have to worry about having a perfect seal since dust won't go in anyway. You would need to have your rads external though or you will have overheating issues (they would just be dumping the heat right back inside the semi-sealed case).