A.P.
If you want something that is EXTREMELY safe, highly-tested & narrow there is only one way to go: Sunshine Kids Radians.
They are steel-framed seats, so they weigh a TON but they are also some of the best seats on the market. (As a CPST-in-training, my top-picks are Britax, Radians & Recaro, in an ideal world & budget).
Good luck!