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.
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/resources/) at 50% infill (ABS) this design is rated to withstand the 550 lb load that the paracord is supposed to break at.
Everything was printed at 30% infill and it worked just fine, but I wouldn't recommend any less than that.
Make sure you tighten everything up really well, my cord kept slipping between the pulley and the body.
All together you will need:
(6) 3.5" long 5/16-18 bolts
(6) 5/16-18 nylock nuts