I don't sew so good, but I can describe ours. There are holes in the sail cover
at the three locations where the Dutchman lines pass into the mainsail
. There is a zipper from each hole vertically down one side of the cover, a couple of feet. Each hole is hemmed with a little extra material that wraps around the Dutchman line so that the sail cloth isn't exposed to the sun.
It seems to me that the key step is getting the holes / zippers in the correct location, aligned with the holes in your mainsail
when it's stowed on the boom. Oh, and I guess you could use velcro in place of zippers.