I did some test-fitting yesterday.
I took a bunch of pictures but will need some time to go back through them all.
I tested:
- Peg Perego Viaggo Flex 120 (high back booster, no option to remove the back)
- Maxi Cosi Rodifix (high back booster, no option to remove the back)
- Diono Monterey 4DXT (can be used as high back or or backless)
- Graco Turbobooster LX (can be used as high back or or backless)
- The two seats I tested that a kid can use without any kind of seatbelt extender are
* Peg Perego Viaggo Flex 120
* Maxi Cosi Rodiflex
I misread the owner's manual originally. You aren't prevented from using rigid LATCH with a booster to keep the seat in place, but you can't use LATCH only with a heavier kid. With that being said, using rigid LATCH on the Peg Perego or Maxi Cosi prevented my 5yo from being able to buckle herself in. Using rigid LATCH just positions the seat in such a way that the buckle is obstructed. In 6 months, she might be able to do it when she gets a bit bigger and stronger.
Both of those seats work okay without using the rigid LATCH and my 5 yo could buckle and unbuckle herself. Unfortunately, they are both expensive ($200 for Maxi Cosi and $300 for Peg Perego). They both have an extremely narrow base - 12" or under - that lets them fit in the narrow space in the MS rear seat.
This might not apply to everyone. My criteria were finding a booster where I could use LATCH to keep it in place when a kid isn't in it, and a booster where a 5 yo can buckle and unbuckle herself without using any kind of extender.
For now, I will keep my 5 yo in her front-facing Clek Fllo and my 7 yo will be in the Peg Perego since I already had it. In 6 months, when the 5 yo truly grows out of the Fllo I'll switch her over to a booster and reevaluate then.
So still looking for a booster (high back or backless) that has a very narrow base - 12" or under - and non-rigid latch...I haven't found it yet!