DS (6) has a hipod. I bought it when he was only 3, & used it with a H harness. Back when I bought it I found a convertible (5pt harness - booster) seat, one of the first out, but it was recalled with in weeks of me buying it. I couldn't find another one under about $500, so went with the hipod. Glad I found one for DD, she's still in the harness at 3.5.

The hipod is quite narrow. The back of it adjusts & moves up, so there are no big wings. The base is probably a little smaller too. DD's is infasecure. It is quite bulky in comparison.