The base is not required to achieve a safe and secure install. The lap belt routes through the guides on the carrier that are located near the carry handle and the shoulder belt routes behind the carrier through the guide located above the carrier release from base handle. Reference the MESA MAX manual and vehicle manual for seat belt installation guidance.
Ensure the red line on the side of the carrier is parallel to the vehicle floor rather than the ground outside the vehicle. Ensure the back angle of the carrier stays in the correct position and the carry handle is rotated forward to the 4th position.