To begin, you need to route some circles out of some
kind of stiff material. Plywood or particle board seems to be a good
choice. They need to be the diameter you want the inside of your tube
to be. If you need to be exact for some reason, you will have to take
into account the thickness of the plastic you will be using to cover
the disks to form the mold. Note the size of the center holes. These
become improtant later on when you go to collapse the mold inside the
tube for removal.