use a very stiff cardboard on that vertical space. mix fusionfiber in consistency so it sticks to a spoon when held upside down. it will spread on cardboard. You may need to do in sections so you can hold hand against back. Use adhesive drywall tape to hold sections together.
Anther way is too spread on pre cut cardboard first , add some adhesive drywall tape tabs, wait a day then while still a bit wet, but firm lay cardboard against the space, then cover seam with more fusion fiber to blend.
getting fancy...
