DD1 has the new Babylove EzyUp booster seat. The Maxi Riders didn't cut it for her either. She's 130+cm and 26kgs. It's got a bit more to go up as well.
I find the boosters for older kids are hardly padded, not like the ones for younger kids. Sometimes I fold a towel up for under DD1s bum and up her back a bit and that seems to help on long trips.