Hi. I recently traveled with my baby from Mexico to California. I used a Kwigy Bo, the Alex bag, and she did pretty well in it. I noticed that my baby was panting a lot, which is really strange for her, so I secretly took her out of the bag and put her on my lap under my jacket. I thought the panting was because she was hot, but I think it was more because she was nervous. I had her in little pajamas and ended up taking those off. On the way going to California she was wearing pjs and laying on a blanket and she wasn't hot. I guess she didn't like her Kwigy too much.
On the way she was in a hard sided carrier. But I would recommended a soft sided that way you can squish it a bit under the seat. Mexicana airlines has an error on their website. They gave the dimensions for the carrier to fit under the seat and it was wrong. The carrier was too big. Luckily the airplane was empty and she was able to ride in the small aisles where the passenger's feet go. It was such a hassle and stressful time for me because they wanted to put my baby in cargo. But definitely get a soft sided carrier and maybe call the airlines to triple check the size specifications. Good luck!