My DIY project is replacing my 100mm extraction fan in my bathroom for a 150mm. It's a through wall extraction fan with weatherboard on the exterior. The size means I will need to somehow make a flat surface on the weatherboard to mount the exhaust side to. Has anyone got any tips?

I've already bought some 150mm Manrose standard fans

I've been told to use cardboard to get a profile of my weatherboard and then get a piece of wood cut to fit like a puzzle piece on top of the weatherboard.

I also watched a youtube video where they cut a square out of the weatherboard and didn't bother with the cut out.