You won't be able to keep your mini pantry full. DON'T TRY TO! A sometimes empty mini is an important onsite control that keeps stewards from policing and shoppers from loitering, etc.,(not to mention you'll wear yourself out trying to keep a small box full in a high need environment).
Intervene if your mini is always empty. Use social media. Take a photo of your empty and ask folks to give. If you’d like to build up a surplus, host a food drive or a door-to-door food drive.
No matter its state, the mini pantry is about compassion, generosity, and trust for all. What you can do is enough.