Second attempt… I was disturbed by the lack of an obvious way for it to open 🙂
It was worth going through this as I’ve learnt more about the mirroring modifier – particularly how it works with the object origin.
I’ve also realised that I didn’t need to apply the mirror modifier to the box so that I could have the hinge and lock flap different on the front and back; I just needed to have two different objects with their own different mirror modifier.

