How to encourage donations after the auction closes?
The donation always stays open.
You can change the Auction Closed pop up message to include a note about the donation remaining open. Edit it at Branding > Settings, then see the field in the Custom Text section.
Go to the Communications section to prompt participants who didn't win in the auction/donate/ make a purchase to make a donation as part of the end of auction messaging. (Note that the Communications Section is not available on some pricing options. See the Pricing options article relevant to your location for details. )