There are photos in my first book (Fine Gunmaking) of a large cavity, before and after plugging, in the butt of my Fox and a couple of shots of bored holes in a story about balancing in Double Guns and Custom Gunsmithing none of which are readily available for internet posting.
Both were in Shooting Sportsman columns sometime back.
I'd recommend reading the stories before hollowing the stock, I'd be happy to provide the books.
Best,
Steve