Below is a video that shows how to build an inexpensive maize ball (often known as a slip bag):
Commercial models average between $20 and $25. One example can be seen here.
For a brief demonstration of the maize ball, look no further than Mike Tyson. Scroll to 1:18 and 1:58 within the video below:
If you enjoy the old Mike Tyson training footage, here is the first part of the video above:
For more do-it-yourself equipment ideas, refer to the following link: