My son is growing up and am looking for a no-back booster seat to replace the big-n-clunky baby one - What brands have worked for you guys with a 3-4 year old? I'm finding it difficult because of the way the back seats are designed - a big well for ur ass, which means narrow tall bolsters - I need a booster that will fit inside those dimensions w/o riding on top of the side bolsters - I hope I was clear enuf