has the Bubble Bum brand of inflatable car booster seats on special at the moment.
This may be the "solution" for travelling, car pooling, visitors etc. It is ECE Regulation R44/04 approved (group 2-3 so 15 to 36 kg)
Regular price is CHF 47.90 on sale for CHF 28.70
. It is obviously more expensive than a regular Styrofoam one but the small size when deflated might be of interest to some people.
The height of the tube (purple photo) is around 16cm so that might help with the size.
Video (EN) and photos.