For donations dropped off at a unmanned drop box, is there a way to satisfy the IRS need for acknowlegment? What would I need for documentation since there is no one there to give us a receipt?

I have, from time to time, taken boxes of clothing items to a Salvation Army or Goodwill drop box.  I have also participated in the donation requests by such places as Sleep Train to get items such as new shoes and pajamas for Foster kids and have put those items in their drop box.  No one is ever around to give out a receipt.  So, what would I need to keep as documentation of the donation?