Can you install the MESA without the base?
The base is not required to achieve a safe and secure install. The lap belt routes through the blue guides on the carrier that are located near the carry handle. Reference the MESA 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 so the baby’s head is not flopping forward and closing their airway.