When I made my DIY SWT recently I put tensioners on the 4 corners. However, I only used a single piece of bungee cord rather that doubling it up. I tied one end to the D-Ring with an Anchor Hitch
(or Anchor Bend) and a Double Fisherman's Bend
to connect the other end to the tie out line about 10-12" out.
It has worked quite well so far.