By 'Loose' do you mean that it slides a bit in the horizontal plane....so left and right ..or is it 'loose' in the vertical plane up and down ?
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Left and right can be sorted by using some Very Thin plasticard. Remove the toothed plastic wheel that makes the turret turn and then measure the gap between the turret and the hull when pressing the turret backwards or forwards. Cut a thin strip or two of the plasticard and then glue it around the inside of the opening in the hull where the turret sits.....This will be trial and sanding down or adding a bit more.
Loose up and down can be cured by measuring the movement with the turret mounted in place and then removing the toothed plastic wheel and VERY carefully sanding down the mounting pillars that the wheel screws onto. Please remember that you need some 'free-Play' to make sure that it can move without binding.

Alb.