I am sharing an overdue picture of how we put the infant carseat into our box bike! The previous owner had installed these things - U-hooks? D-bolts? - in the bottom of the box. There's one on the other side too. While it was a tight fit, we could clamp the carseat base right into the box using its U.S. standard "LATCH" system clips. The seat itself then snaps into the base (facing backwards, of course). It can also be used in a car without the base, so we just left the base on the bike and buckled the seat in with a seatbelt on car trips.