As mentioned in another posting on the same subject, you could drill holes through the existing plastic fridge lining from the inside of the fridge compartment and into the otherwise inaccessible space between it and the structure forming the outer part the cupboard spaces. Then pump expanding foam through those holes, and finally seal the drilled plastic with white blanking plugs of the type provided by self assembly furniture manufacturers for covering over unused holes or screw heads etc. You might need to be careful not to overdo things, avoiding squirting too much expanding foam through those holes, as you don't want the pressure from the expanding foam to do damage to the furniture as a result of over pressure from the foam.
