You could frame the inside with the insulation between the 2 x 4's. Do a frame for the top to fit in the top. Put the insulation up and cover with Masonite or paneling. That should hold it in place.
You could frame the inside with the insulation between the 2 x 4's. Do a frame for the top to fit in the top. Put the insulation up and cover with Masonite or paneling. That should hold it in place.