My favorite short game training aid would be a hula hoop. Old school I know, but placing it around the hole and having a player focus on getting their shot inside the circumference of the hoop is great visually and mentally. Great confidence builder. I also have each player hit these short game shots with clubs they would normally never use. Six iron, putter, all the wedges and even a hybrid pending the situation. The ability to execute the shot with an odd club builds even more confidence. Short game is all confidence.