I tend to "downgrade" my kids' seats for travel, so the (now) 6 year old used a backless booster for our cruise last summer. He's already booster training at home in our second vehicle....sometimes with a back, sometimes without. We took a car service to/from the cruise terminal last summer, so I got to watch him the whole time, and remind him to sit properly.
We carried both backless Cosco Rises in a reusable shopping back, and stored them at our youngest's feet on the plane.
None of the foldable or inflatable booster seats are rated very highly by the IIHS. I've tried both the Bubblebum and the mifold, and neither are all that easy to use IMO. Specifically they've been hard for my kids to use on their own.
The Cosco Rise is cheap, lightweight, and my kids can typically buckle themselves with proficiency.