First let me say this: This is FANTASTIC, and where-ever you wear it, I hope I get to see it, if not, I want to see loooooots of pictures.
Now, moving on. You seem to have a good idea about how to go about it. I am new to things outside of a needle, thread, and fabric, but I think that could work. Also, like Kitsoru said, something semi-flexible would be a good idea. Maybe a thin like plastic, it can hold a form, but thin plastics like Wonderflex can be molded, and are semi-flexible.
I cannot wait to see it!
2012 Convention Plans
Dragon*Con: Tinkerbell (Peter Pan), Cordelia Chase (Angel), Jessica Moore (Supernatural), Red Playboy Bunny, Belle (OuaT), Crystalia Amaquelin Maximoff (The Inhumans), Abigail Brand (X-Men)
NYCC: Kitana (Mortal Kombat 9), Scarlett (GI Jo), TBA
Super Mega Fest: Princess Python (Serpant Society), TBA