Why not use a foam core coated with bondo? I'm not personally familiar with paper mache so I can't comment on if it's efficacious, but bondo tends to look very solid and weighty when finished, it's sturdy, and is great for carving detail into. Just apply it patiently and sand it smooth, then work in all the details you need with a dremel. Fair warning: I've never seen bondo used on Styrofoam (usually it's EVA foam) and it creates heat while curing, so if this sounds appealing, I'd test it out first on a spare section of foam.
"The important thing is no one was hurt... No one who counts."