It might sound crazy, but consider deleting your other retainers. If you delete a retainer they'll all move up a slot so you can access them like that (i.e. if you have 4 retainers and delete retainer 2, then retainer 3 becomes the new retainer 2, retainer 4 becomes the new retainer 3).

It's far from perfect, but mentioning it as it might be an option for you.