We've been using a Kobipet carrier for our 5-pound Gracie. She's been on 4 plane trips in it and it's working so far. Here's the link:
Kobi | Pet Carriers for dogs and cats
She can't stand all the way up in it but she's got lots of room to move around and lie down. It has mesh on both ends and on top so when she's under the seat at my feet she can see me. I can also pat her on the head to calm her down on take-offs and touch-downs. She gets a little Dr-approved tranquilizer but I don't give her much and she does get nervous.
I also saw someone on my last flight with a wheeled carrier that had a lot of mesh - when they were wheeling the pup in the airport it was sitting up completely and looking through the mesh. I think when they lay it down under the seat, the whole front and top are where the mesh is. I think one nice thing is if the pup can see out well and see you.
Good luck.