when you take your head unit out there is a bracket on each side
(that is held in by 4 screws, i think). its basically a flat metal plate that is the same size as your head unit.
you need to take that off and reuse it with your new head unit. as far as the universal din pocket goes i'd recommend you get a legacy din pocket with the door because it looks really nice. or you could just get a normal subaru din pocket from a dealer for free/cheap which will be made to work with the bracket in your car. i'm not sure how to fit that universal one.