Thank you for your question, Christine.  Since there doesn't appear to be any dangerous tree tissue remaining, the best advice is to do nothing except provide adequate water this summer to keep what is left healthy.  A healthy tree will naturally try to scab over the wound, so don't apply anything over it.  You will need to monitor the tree, since such a substantial wound may invite insects and diseases.  You might want to consider hiring a certified arborist to examine and advise.  Here is more advice:  https://www.purdue.edu/fnr/extension/tree-wounds-and-healing/
Good luck!