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