Commit Poppac’s are perfect to hold 10 AAA batteries!
If you know someone that uses Commit (Stop smoking lozenges) have them save you the blue plastic- highly water resistant POPPAC. 10 AAA batteries fit perfectly into it. Not too snug, not too loose
EDIT: As Trailmaster noted you need to put some non-conductive layer between both "layers of batteries" Anything plastic will work. ex. old butter lid, etc
I don't normally share my ideas but I thought this one was good enough.
My other ideas...
How to protect your Data from Water and Static-EMP Longtermviewtopic.php?f=14&t=44982