Just for fun I designed a block and tackle with the intention of seeing if I could make one strong enough to old my own weight with paracord. (Paracord as seen on Amazon: http://amzn.to/1gIiqlo)
This design used 6 pulleys to divide my 170 lb body weight 6 times into roughly 30 lbs input force. It was strong enough but it was hard to balance! So I later designed the mount and center holder to keep everything centered.
The design is also modular so you could use more or less pulleys on the block if you wanted.
According to my clevis calculator (available here: http://engineerdog.com/resourc...) at 50% infill (ABS) this design is rated to withstand the 550 lb load that the paracord is supposed to break at.