Advice needed - "invasion of the mealworm snatchers"

Hi to everyone - this is my first post . I have been feeding a wide range of garden birds sucessfully for about 18 months.  I have not had many starlings in the garden - until now !! I put suet pellets/ blocks out and soaked mealworms scattered on the ground and on feeder trays and they have an area in the garden almost exclusively for them.  A robin visits the garden throughout the day and I am chuffed that I have almost got him to feed from the hand .As I wanted to keep on providing separate mealworms for the robin to ensure he keeps visiting and hopefully will have the trust to come fully to hand, I also put mealworms in a gaurdian feeder. However, unbelievably , the starlings can squeeze through the holes ( they are about one and half inches by 2" ) - I couldn't believe my eyes!!  I have had a look  for another gaurdian with smaller holes but  I think they would still be able to stick their necks in. Has anyone got any advice on a gaurdian they can,t get into before I buy up the whole of the garden centre? The poor robin looks so dejected when his previously exclusive supply of mealworms are disappearing!

Many thanks