FWIW, my kid is still harnessed and we have a RSTV, but I'd like to just have something cheap in case it gets trashed.
The Turbo is great, but for the purpose of a spare seat for extra kids I like the Harmony Youth Booster which runs $10-$14 at Walmart. It earns the IIHS "Best Bet" rating for fit and it's super cheap and light.
If you do get a Turbobooster, don't forget to insert the screws in the arm rests. Nobody ever remembers, but the seat has actually been known to fly apart in an accident without them.