I decided to carve the two shorter cervicals from a solid block of foam. These are the last two cervicals. The ones which connect to the thoracic vertebrae (notarium). I tried to make one of them hollow but it got too complicated, so I gave up and cut out a block of XPS foam for each one.
And then I carved them in the shape of the vertebrae. I later added some details with strips of foam, filled in the gaps and shaped with fire. Here they are in dorsal view compared to the sixth cervical vertebra (cervical #5, after atlas/axis).
I tried to fit the vertebrae together. Based on how well they connected, I made some adjustments, cut some parts, added some foam. When finished, I covered each one with a layer of acrylic resin (modelling paste) and when dry, stained them with coffee. Here are some pictures of the final result.
This is the front:
And this is the back:
The two last cervicals seen from below, separated and connected:
A view from above:
And from the sides:
These pictures show the last three cervical vertebrae connected. This is a view from below:
Here is a dorsal view:
And a side view: