Looking for an ultralight, large fill material

I was going to suggest the Styrofoam bead beanbag fill material but idkfa beat me to it. Ive used this a lot over the years as fill in rudders and bulb keel blades mixed with the slowest cure epoxy you can get to keep exotherm at bay. You would need a skin over it though as its not something you can just sand and paint so not sure how it would work for your application.

Steve.