Booster / Stage 3 car seat

[Claribel]

As we have these until kids are 12, I want to make sure I get the right one ...

Can anyone provide any feedback on booster / stage 3 car seats that they have liked / disliked, and suggestions of models gratefully received! We do do quite regular long trips at bedtime, so I want to make sure that we choose one that my daughter can safely and comfortably sleep in.

Unfortunately our car does not have Isofix, so I am also anxious about the fact that if the child isn't in the car seat that it will rattle around, any clever tricks for this other than having to remember to strap it in each time? Or do we have to investigate getting isofix fittings (seems a bit crazy to at this stage ... ).

Thanks in advance.