I use Chefade and it works a treat - a dob just outside the edge of the trap and another bigger dob inside at the very back.
Magpies are very suspicious of anything new at first so just leave the trap alone other than checking and rebaiting if the mynas have got to it.
My experience has been that the mirrors make no difference (mine are now all filthy and I still catch magpies without any effort). I’m also not sure whether having one in the cage effectively lures another - I’ve caught a lot of pairs and I’ve caught a lot of singles. I clear my traps once or twice a day.
What I understand to be very important is that when you’ve had a catch, cover the trap with a towel when you go to remove it and also dispatch the captured magpie at a place where other magpies wont see you. I do it in the bush. If magpies see you moving or dispatching another magpie, they’ll be very suspicious of your trap and any kind of bait wont make a blind bit of difference.
I’ve trapped in excess of 100 magpies and my property is magpie free aside from the odd incursion by an opportunistic family which is quickly remedied. I’ve now moved onto mynas which seem to fill the void left by the magpies. Nonetheless, our area has seen a noticeable increase in native birdlife.
Keep up the good work!