Step 4: Cut, Sand, and Fit the Cork Into the CDs
a) you can make them as snug as you need to
b) you don't hurt the turbine's efficiency by running a dowel through the middle of it (which, according to some of my research, knocks it down by something like eight percent)
You can take an ordinary wine cork and cut it in half, then work the pieces over with a saw and some sandpaper until they fit just right into the CDs' holes.
Be sure not to over cut. Cutting away the outside of the cork is a great way to save time from sanding, but don't over do it. Better too snug than too loose. When you get close to the right diameter for the cork pieces to fit into the holes of the CDs, test and sand and test again.
Once the corks are the right size, pierce them with small nails. These will form a "needle bearing" on which the entire contraption will spin. Try as hard as you can to get the nails to be perpendicular to the CD and as close as possible to the dead center or it. Otherwise, you'll get wobble.