pvc piping would work, but DO NOT bake it in the oven with the clay. PVC will offgas toxic fumes when heated. You merely use the pvc as a form.
On the safer side, if you can get your hands on some huge diameter bamboo (3-4 inch), it too could be used as a form. And could possibly be baked with the clay still around it.
A glass vase would also work, again only as a form.
Wax paper placed around any of these should make it easy to remove the clay.