Parts List:
2-Softballs
2-Eye Screws (recommended 7/16")
2-Lashing Straps (you can use rope if you don't care about changing height, or chains if you have q-links or carabineers)
Simply put:
Screw the eye screws into the softballs and hang them from a bar, rafter, tree branch, etc. with the lashing straps. So easy, cheap and effective!
Here's a video of me putting a pair together for less than $10...