I have decided to use a strong rare-earth magnet to attach the turret to the hull. I have mounted a small metal washer to the hull.....
And the magnet is mounted in the turret.
I mixed up some Milliput and placed it into the turret. I then added some superglue to the magnet and placed it onto the Milliput.
I placed a spare washer under the magnet and also a thin shim of .5mm plastic card under the washer before pressing the turret down on to a piece of hardened glass.
When the Milliput had fully hardened, I tried it on the tank hull to check that it worked properly.
The plastic card shim meant that the magnet always stays just a fraction of a millimetre above the washer rather than sitting on it.