How to handle varying mass value in Lightweight Airplane Design

I am considering the aircraft model and compensators used in the Lightweight Airplane Design but with a varying mass value.

1- I cannot find the mass parameter to change it from a fixed value to a varying one.

2- How can I modify the longitudinal controller for controlling altitude (compensator C1 in yellow) and for controlling pitch angle (compensator C2 in blue) -> make the system robust against mass paramter variation.