4″ X 1″ X 1″ corner brace. Glue or staple.
5/8″ X 5/8″ corner brace.
2″ X 3/4″ X 3/4″ corner brace.
2″ X 3/4″ X 3/4″ corner brace
3″ X 3″ X 7/8″ corner brace