This shows how the Grill secures the rubber gasket. the locking tab from the grill fits through the opening in the gasket and then the Punk housing.

Front grill and gasket removed and now the slots for the locking tabs for the grill are fully visible

With the gasket and the grill removed, use an opening tool (i.e.guitar pick) and insert it between the halves of the Punk housing. There is some glue between the case halves, so a bit of pressure and wiggling maybe needed.