My plan was to develop a prototype, then beef it up once I had it figured out, but it has lasted 3 years, so far.
Objectives included to be able to disconnect when not using so Sail cover didn't need modification.
I'll try to describe how to build.
There are two parts: the cradle that holds the sail when it drops, and the support lines which tie off to the spreaders.
Materials: 3/8 inch (or so) white polyester braid, about 50 feet. 3/8 inch bungee cord in bulk -- I purchased about 15 feet. 4 Bungy hook ends for 3/8 bungee. A hand full of small nylon tie wraps.
Note that larger boats with larger mains may want beefier bungee for the added weight, and may need longer bungee and line lengths.
Create a long bungee cord (mine is about 9 feet) with hooks in each end. This is doubled, and attached to the underside of the boom. I was lucky to have existing hardware for attachement: Topping lift cleat forward and mainsheet bail aft. I looped the bungee through the center of the cleat then secured the ends to the main sheet bail. You want the line to be relaxed, but not drooping down from the boom. This will form the blue line in the photo, when deployed.
Using a tie wrap, tie the two bungee lengths together under the middle of the boom. Combined, this bungee will form the bottom of the cradle with four support lines on each side, shown in blue in the photo.
Next, cut two lengths of 3/8 line that are half the length of your doubled bungee, plus about five inches. These will form the upper portion of the cradle.
For the starboard side (will repeat for port side), tie a loop in the line about 1/3 from one end (see picture) that will form a loop at the top of the cradle to attach the support lines. Then tie a stop knot in each of the free ends. Position the line so that line length is about centered on the boom bungee with the loop and short end toward the mast. Secure the line ends to the bungee -- ONLY ONE SIDE OF THE BUNGEE LOOP -- with a tie wrap, pulling it fairly tight so that it doesn't slip. We will give it the final tighening later after everything is adjusted.
Repeat for the port side, tying the line ends to the OPPOSITE bungee loop.
Now there should be bungee and lines all lying along the bottom of the boom. If you pull up on the loops either side of the boom, you should see a cradle form like that in the picture. For the support lines, I simply tied the high ends to the spreaders (near the mast) with 3 or 4 half hitches. At the lower end I created 12" bungees with hooks on one end and loops on the other and fastened the line through the loop with a sliding knot for adjustment. You can see these at the top of the picture.
You can tweak the final fit by adjusting the support lines and sliding the tie wraps as needed. Once it is all set, tigheten up the wraps hard and cut off the ends.
When you are ready to put the sail cover on, simply unclip the support lines and tie them off at a convenient spot on the mast. I clip them to the halyard exit holes that happen to be at the right location. The cradle will snap back into position under the boom where it is protected from the elements by the sail cover.