This is the only portion of my Haunted Railride that I could copy without all kinds of surface stuff. You would have to build rails going to and from the underground entrance & exit.
It is a large room (101 x 151 x 45 inside) with lava floor and glowstone ceiling. The glowstone is covered by iron trap doors. There are also jack-o-lanterns in the walls and here and there for lighting.
The room begins on the wall opposite the dragon head. Exit is through the dragon head but then goes along inside the wall back to somewhat close to the entrance.
There are frozen entities hanging around, a few little rails with mobs riding, and sound effects. There are also a few fireballs just missing the rider approaching the dragon's mouth (fireball coordinates will require updating)
Primary materials: nether brick and netherrack