gnome boxes stores its disk images in

~/.local/share/gnome-boxes/images/*

I found that libguestfs-tools is easier for mounting your preferred image to a folder and get the files from there.

mount your disk image:

mkdir ~/path/to/mountpoint
guestmount -a ~/.local/share/gnome-boxes/images/boxes-img-name -i --ro ~/path/to/mountpoint

copy your desired files from inside, then unmount:

guestunmount ~/mountpoint