Success!
The back is a bit untidy, but at least the case top shouldn't flap about once it's put back together.
What I did:
- Stick some blobs of epoxy putty to completely cover where the original case tabs would be.
- Eat pizza while the putty cures.
- Attack blobs with mini drill equipped with cutting disk, drum sander and pointy burr to remove material until I had a shape that would fit the holes in the top case.
I'm quite happy with this as a hack, only time will tell how long the repair lasts.