Exterior room wall that can blend into cave wall

When raising a cave area around a room (or punching a room into a cave area), no matter how tightly you try to bring the cave walls you end up with an air gap between the cave wall and the exterior room wall. It would be nice to have an exterior room wall option that both a) matches the cave style in appearance, and b) is seamless with the cave wall.
This way you can have a finished room, that is flush within the cave.