Those box shaped cutters look absolute tosh. It takes about 20 seconds to cut one out with a straight multitool blade. How many seconds do you need to save per box, is the job that time critical, are you doing that many boxes it would make a difference ?
Using multitool blades in plasterboard ruins them pretty quickly anyway so you're going to need a constant supply if doing a lot with them. You'll need a single and a double which is about £20 combined. I use one of my sub £2 wood bits that is on the way out. Much cheaper.