D.
Hi A.-
I was one of the people who responded before. What ever you want to use would work just fine. Wiether it is a heavy dudity garbage bag or box or cheap duffle bag. The only requirement I have had in the past from Southwest (we tend to fly them a lot-they are a great airline to travel with kids-kid friendly and usually on time) is that your name is on there somewhere and that there is no loose strings. You can check them anywhere you would like inside or outside. I always just look for shortest line. Hope you have a wonderful trip. Love to hear how it goes. D.