Travel Seat for a 4-year-old

We are Americans living in England with the military and getting ready for a trip back home either in the spring or summer of next year. Four airline tickets are costly, so we are budgeting our expenses now, and one of the things we are looking at is car seats for travel. We want two lightweight seats that are FAA certified and in a perfect world, would allow a small adult to sit between the two seats in a rental car.

Our younger daughter will be between 17-20 months and under 30 pounds, so she's no problem: Cosco Scenera. I thought my older daughter, who is short and will be between 4 and 4.25 years when we travel, would also fit in a Scenera but weighed her and found she's 37lbs already. I doubt a Scenera will work for her next spring. While she is very mature, behaves well in the car, and rarely falls asleep in cars, she will only be at the barest minimum age/weight for a BubbleBum. Her main ride is a Britax Frontier 85 so if we can't find anything we'll use that, but I would prefer to use a seat with a smaller footprint on the plane and in a small rental car. Though we have a CARES, I'm not comfortable checking the Frontier with our luggage.

With a max. budget of $50, what would you suggest that we get for her? What fits in the budget travel seat window between just-barely-too-big for a Scenera and just-barely-big-enough for a BubbleBum with a 5pt harness? Or is the BubbleBum appropriate?

Maestro maybe? It's fairly lightweight...

I think a Maestro is your best bet, although even on sale, $50 is a little low. I think I heard about people getting them for $55 recently, though.

How much use will it get? What kind of driving?

Someone has a Maestro on the SWAP on here I believe for $50 shipped, I saw it yesterday.

Thanks, all! I hadn't heard of the Maestro. It seems the Maestro won't ship to APO addresses via Amazon, though. Pout. Hopefully that idea helps someone else. Any other ideas? Other retailers?

We plan on using it for our flight (England to Boston in an Airbus), about 2-3 weeks of around-town and highway driving in rural and suburban New England, with possibly a side trip to DC via Southwest Air, and then to return home to England. After that I'd probably sell it to an American family on base in a similar situation. If we can sell it I don't think I'd mind the $55+ price tag, though that isn't quite ideal. We are looking at buying a Ford Flex next year through base sales so I suppose we could hold on to it for guests or for a second car, but we are trying to remain a one-car family. I really wish the BubbleBum was an option, but maybe for our big trip back when she's six.

What about one of those Ride Safe travel vests? They're legal for use in the US and would fit a child that size, wouldn't they? You can't use it on the plane but she might be big enough for the seat belt only on the flights (I think 4 years and 40 lbs generally fit most airline seatbelts).