Perhaps a bit odd coming from me, but being a Phantom cosplayer myself, I wanted to see if I could help and throw my two cents in there. By the look of the concept art, I would think a strapless white bra could work.
There are several cosplayers who have done this costume. I would recommend searching for them via costume search and asking them what they did.
*The Phantom of the Opera: The Phantom/Erik/Red Death(2004 Movie Version)
*Prince Eric (Disney's The Little Mermaid)
*Pirate (From Pirates of the Caribbean)
*Orlando Bloom/Will Turner
*Haunted Mansion Stretching Portrait Quick Sand Character
*Oscar--The Great & Powerful Oz