Although it looks like a pain to make, it was really quite simple.  I picked up the tetherball set at the toy store,  got an old tire from a local gas station for free, and a few supplies at the hardware store.  Once you are done, you'll just have to let it sit overnight to harden, and then you're ready to play.

Shovel

Lay the old tire down on top of a plastic trash bag.  Make sure the bag completely covers the area under the tire.

Pour the bag of cement into your plastic trough and mix with water according to the directions.  Use the shovel for mixing.

Once the cement is thoroughly mixed, let it set a few minutes and then shovel it into the center of the tire.

Let cement harden overnight.  

Attach rope and tetherbal, roll to location desired, and you are ready to play.