I've never found a perfect solution for plastic, mostly because it depends on the type of plastic I think.
For the hard plastic of a notebook case, and a join that's probably gonna be under some stress, I'd try epoxy. Consider sanding it a bit so it grips the plastic better, and clamp it down to apply pressure as it cures.
I make zero guarantees about this approach, like I've said, never had a good time with plastic.