I've had luck pumping something like this into the frame rails of cars to reduce road noise.
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This is basically a pourable closed cell foam. You could easily pour it on flat surfaces. If you're creative you can cast it into or onto 3d surfaces. It's easily sanded and you can seal the top with epoxy and/or fiberglass. The 8lb is close to balsa and the 26lb stuff is close to the density of poplar or pine. It also is an excellent adhesive. It will stick to just about anything that isn't greasy or waxy.