For our boats, my wife and I laid fabric (we used rip stop nylon) over the cockpit and clamped it to the combing lip. Make sure it's tight in all directions (we used 8 spring clamps). Now draw a line on the fabric where the top inside corner is under the lip. Remove and cut out at least an inch larger than your line. Fold over and sew to make a pocket for bungee (1/4" shock cord) having your line as the new outside edge (this where your bungee will pull tight against your combing) and leaving an opening in the back to insert the cord. Work bungee through pocket and place over cockpit. Give a good tug to stretch the cord and tie off. Hope this makes sense, it's really quite easy.

David