Confidence: It's likely to be this, but I can't be certain.
Notes: Using the sharks trust identification guide for mermaid's purses (linked below) I think this one belongs to an Undulate Ray. While the rough edges seem to be gone on this one, the capsule is too large to be a spotted ray case at 6.5cm. There are small remnants of rough edges but no keels so it isnt a thornback ray. The outwardly pointing proximal horns are also correct for Undulate Ray. Undulate and spotted ray egg cases are said to be similar but this one appears to have the features and size of the former. It has already been soaked in water overnight.