Anyone know of a good high back booster car seat for tall child with broad shoulders? We bought a Britax Evolva for our tall son when he was a baby, but it doesn't fit well in our Berlingo and we want the next one we buy to last our tall son (128cm at 6 yrs) for as long as possible. We are considering the Britax kid plus but are worried that he will be too broad for it very soon. Any suggestions?