Hi Xan,
I think you can use a diamond file and remove the lower sides of the flat shank to approximately 0.5 mm width.
Then drill a 0.5 mm diameter hole at the correct angle into the model surface.
Secure the turnbuckle into the hole using CA adhesive.
This should secure the turnbuckle in the model surface, but also leave some of the flat shank visible, as on the real aircraft.
Mike