Two guitar picks did the trick – as soon as the edge of the first pick goes into the gap, twist it more upright; the next pick can then be slid into the opening and the rest of the case opens up easily. One warning: be very careful near the charging port – there’s a delicate plastic trim around the opening in the metal case and I accidentally cut through the trim with the edge of a pick. The trim is attached to the case with two screws so I removed it before repairing it with superglue. This worked well and the trim screwed back in very easily.