In order to reproduce the "Pacman" shape seen in the reflected beam (see #entry 687), I have made a simulation of the cavity using the FFT code OSCAR.
https://fr.mathworks.com/matlabcentral/fileexchange/20607-oscar?requestedDomain=true
The shape is more or less reproduced with a mismatching of 4% and a misalignemnt of 5microrad of the end mirror.
I attach the plot of the fields and the script used (it needs the code OSCAR to be used), which can be useful also for other applications.