J.F.
We used one of our Cosco seats (not sure of the exact model) and it sat our daughter too high for the tray table to lay flat. We used a portable tray that we bought at Walmart for like $15. It is soft and light and can be used in the car as well. It has a lip around the edges to keep crayons etc. from rolling off. We don't use the portable tray often, but take it with us on trips in the car and airplane.
Good luck!
Updated
I remember the airplane buckle being an issue, too. We put a blanket between the airplane bucke and the seat cover. Something to consider. We also needed the carseat when we got there and didn't want the seat to be thrown around in the cargo hold and we didn't want to rent a seat from the rental car agency.