J.
Hi,
I bought an Evenflo high back booster for about $60 for airplane travel. It's fit in every seat (although I haven't been on a tiny prop plane with it) and it "relatively" light to carry in a backpack style carseat carrier. It's also worked well in cars, but it seems to fit more snugly in the seat if you attach it with the seat belt rather than the latch system.
J.