Extra booster for third graders

Since DD moved out of her into her Monterey, I'm down an "extra" booster. I have a backless Grace TB that I use when driving another child and there's a BubbleBum I have available, but prefer to bring only for our own emergency travel needs. I want to get one more backless booster to use when I have a third child in the car with me. This would usually be a third grader (so 8 or 9 year old). What's the best seat to get for these somewhat larger kids?

ETA: I forgot to mention that we're in a Sienna, so the "third kid" would be in the third row...but there are no other carseats to contend with.

The Evenflo Amp is a nice one for bigger kids, good depth, easy to buckle, cool colors, stable. It's a little more than others that are super cheap and light like the Harmony Youth at Walmart, but that one isn't my kiddos favorite, due to a slight tippy feeling and shallow seat. Though it does have some extremely cool covers that might well make up for all that (my girls will sit on granite, as long as it has pink zebra print...).
So I'd say you could go either way, even another backless Turbo, really (though if you're working with bigger kids, it's a little tight in the hips, I imagine you are past booster law age, and want to make everything as pleasant as possible...? Most kids are perfectly used to the Turbo, and it does come in fun colors...). If you have a BRU near you and a trade in (any old carseat, till 9/30, 25% coupon), you could get a Turbo or Affix (which is a tad deeper and has LATCH, hence the higher price), for a pretty decent price.

Super-helpful. Thanks! Yes, getting pushback from some kids at this point, but, hey...my car, my rules. If there's something that can keep them safer, I'm going for it.