Hi again,
I’ve made some tries with a clone camera and modifying it’s viewport (see this playground test), but it seems that it is not the proper way to go about it…
The only interesting resource I could find on how to resize a camera is this one, so I wonder if @RaananW may have some clues on how to achieve what am I looking for? 
Especially since the project I am working on uses a XR camera, and you seem to know how I could create an appropiate projection matrix (own deduction from your answer) 