The panelling to the interior is textured lollipop sticks. Very little of this will be seen, but I thought it was worth including as it will add strength to the cardboard structure.
This final image shows the forge alongside one of my earlier scratch-built buildings, a Wooden Hovel which has already featured on this Blog. The Hovel was built from textured lollipop sticks over a cardboard core.
As with earlier posts, a Build Thread can be found on the Lead Adventure Build Something 2015 Contest page, for details go to this link.