Pretty simple, if you can't tell what I did, simply sculpted some putty inside the Tony Stark head and periodically checked on it, keeping it loose so it won't bond too tightly on the neck peg. Flask is from a buffy figure.
Good work on the head sculpt, the the proportions are crazy! Get ahold of a Movie Masters Joker Goon (or even the Scarecrow, since it's the same body). It's much smaller than the DC Universe figure you used. You could try using the coat on that, and trim it from the bottom-up once on the Joker Goon body. Cheers, -Blayne
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Cheers,
-Blayne