I've done expaning foam with plastic on top for my Raven sword and it's pretty damn light. The entire sword was about 8 lbs, and it was mostly due to the wooden core. I think my only advice would be to use glue a bit more sparingly than I did. I used a TON of Liquid Nails on it because I didn't think the weight would add up, but it wound up adding a pound or two to it. I have the full sword build on my blog if you want to check it out.
I've seen people do big stuffed pieces with polyfill and fabric, so hopefully someone comes along that has experience doing it. My only problem is that if you don't do it well it turns out looking wrinkled and weird yknow?
Best of luck to you!