The hardest would be to make the cups. But it will be much easier to have an already existing cups and cut it open, then insert some semi-angled grill over it ?
After thinking about a little more I don't think it would be very simple, my reasoning is that when building a SennGrado or other wood cup style Grado like set, the chamber dimensions (inlcuding thickness of the walls of the cup) matter a great deal, location of the driver within the housing along with how they are mounted can have different effects and not all of them good.
But I still would love to see someone try, just a little testing and tuning for me to take on.