Not sure about the brands you guys have, but we have DD2 in a half seat booster (she is 5.5) & DS in a hi pod. The hi pod is adjustable.
We have the H harness, but DS refused to use it, so I have just the seatbelt, but with that one it fits into the seat to fit across him properly. I am going to put the harness back in though & see if I can make him use it. He threw a tanty & stood up the other day, so I need to do something.
I dunno, I should never have given up the fight in the first place