Backless Booster Seats

[Artechim]

From the SA govt site:
Booster cushions – As booster cushions do not include a backrest and sides they must only be used in the centre position of the car as they do not provide any side impact protection for children. They are generally not recommended as full boosters provide higher level of safety.

I use one for DS (5yo) but we are in NZ.

[Amaunet]

I have one from Kmart as my extra/occasional seat - as above, I only use it in the centre position- between two other seats.. If the centre seat only has a lap belt, you can't use it at all. Kmart also had a folding booster with a back, no tether, they were quite narrow and reasonably priced.