I have a Nacra 5.0, similar boat. I put little blocks on the rear beam lacing under the tramp on the corners. Shock cord runs down through tramp, diagonal back to rear beam block, across along rear beam to the other block, then diagonal across and up through grommet to the other trap line. It takes 22 feet of 1/4" shock cord.

This long length has 2 practical results. First, when the top two feet of bungie gets rotten in the sun you can cut off the 2 feet above the tramp by pulling some of the long length and reattaching it. You'll never notice the change in stretchable length. Second, and most important, when you take that inevitable trip on the wire out to visit the bows the bungie won't break. Don't ask me how I know.