Is it "obvious" to patent a shape, when applied to a device?

I have built an enhancement to an existing device. I would like to patent it, but I feel it it may fail the obviousness clause when applying for a patent. This is the best analogy I can think of: Can I potentially patent a box shape when applied to the head of a rocket? In this example, the box shape has specific advantages (over other shapes of rocket heads) and no patents for box-shaped-rocket-heads exist (in this example, no one has thought of it).