We use something like this for our 2.5 yr old and have for a while. (Graco Platinum Cargo Booster Seat) Our is very stripped down, so most of the padding shown is not used.
http://www.target.com/gp/detail.html/sr=...%3AGraco&page=1We used to use the Britax but agree, it's pretty big. Cosco (brand not the store) also has similar styles.
That being said, this type DOES NOT WORK on airplanes. The seatbelt is looped just underneath the back padding. This is OK when you use a car seat belt that buckles on the side. But on a plane, the buckle is on your tummy. On a car seat like this, it translates to the middle back of your kid. (Hope this makes sense.) It will not work, no matter how many creative ways (extenders/pads/etc) you try. Suffice to say that was a wasted effort bringing it back and forth to Mexico.
At 20 lbs, sounds like your kid might be too small for the true convertible booster.
For plane usage, you really have to think about where that seatbelt is routed because the seatbelt buckle is a major problem.
