How about a different tack? What about putting up some secure nesting boxes? You can buy nesting boxes with metal rims to the access holes to stop attack by predators. If the access hole is small enough then the magpies (or squirrels, etc) will have difficulty in getting the chicks out and this would protect them until they are fledged anyway.
I had a nest of robins which was in some ivy in the garden which were being eaten by a jay. I despaired of what could be done to help them as one by one the chicks were being picked off. Eventually there was only one chick left and I put chicken wire over the ivy and around the nest. The adult robins could get in but the jay couldn't. The remaining chick survived to fledge.
Mind you, that was years ago, now I wouldn't interfere.