A strong 2" wide adjustable Celltei® car seat strap to buckle the Pak-o-Bird to the front passenger seat.
To be fair, I haven't used mine because I realized I can put the seatbelt through the straps in back of the carrier and it does the same thing. I'm sure if I did use it, it would work fantastically as it's made very well.
Now my parrot can travel with me safely!
I've found that placing my bird in the car facing sideways works best. If placed the way shown in the photo above, every time the driver accelerates or slows down, the bird will be pitched forward or back with no way to brace herself.
Also I'd never place my bird in the front seat where the airbags might deploy into them. Plus if your car is part of the massive recall for defective air bags, the front passenger seat is the most effected.