This isn't good. Your irrational (I mean no offense) fear of flying is pretty severe and it would seem that a pharmaceutical solution is not going to be a complete solution. 2mg Alprazolam (Xanax) would be considered a rather hefty dose for someone with no tolerance. I can only assume that it's very important for you to get on that plane, so you need to come up with a battle strategy.
You clearly are going to need to anesthetize yourself to make this work, but you also are going to need to do some major positive self-talk to make this work. The thing is, these people at the airport do not fuck around, as I'm sure you've heard, so you must be coherent to a certain degree just to pass through security, nothing crazy, but you need to be able to remember your name, your gate and other pertinent information. For this reason, I would suggest taking no more than 4mg total Clonazepam prior to the flight.
This suggestion is based only upon the fact that you implied that 2mg Alprazolam "did nothing". The issue with Clonazepam is that it takes a slightly longer period post-ingestiong to experience effects. If you experience effects from Alprazolam within 30 minutes, expect approximately an hour from Clonazepam.
Are you flying by yourself?