What kind of seat does your 5yo LO use in grandparents car for occasional use?

DS1 is tall. Currently in a Graco Nautilus in my car. He was using a Graco convertible in my moms car, but we got that back to use for DS2. Ideally we would like something a little lighter. It will be installed with the seatbealt since he’s right at the limit for LATCH.