Hi DLuders,
I don't have any concrete idea about battery box, but I think it would be better to choose rechargeable battery cell.
9V battery is too big and heavy, isn't it? and small disk battery is too weak and expensive.
actually rechargeable battery needs its charger, but it sounds good solution for heaviest users, children.
Your PF-Old electric parts adapter looks nice!
could you use 9V battery box for PF!? so nice.