Does or did the car have a tow bar? If so, use the original tow bar bracket under the rear of the boot to mount said tow hook.
If not, pop the boot and have a look down the rear panel, you'll see a short diagonal brace from the boot floor to the rear panel. I think that would be a reasonable place to run a tow hook from. Jut make sure you fit a decent size plate behind the panel to spread the load. 3-4mm thick plate 75-100 mm square should be more than enough I think to reinforce.