I had so many projects going on when I first got Monita I didn't have time to make a main sail cover. I ordered a stock cover from Sailboatowners.com for a fair price. I considered it reasonably well made and it did its job. I like the Velcro lined flap that covers the zipper to protect it from UV. I did not care for the the particular way the neck of the cover tied around the mast or the attachment at the end of the boom. After a couple of years there were a few spots that seemed worn, and the unsealed fabric edges of the center seam were fraying badly. So this winter I dug out the sewing machine and made a number of improvements.

Edit 2/14/21 Minor winter projects: To make it easier to get a grip on both sides of the cover to get both halves of the zipper lined up and started I sewed a couple of tabs to the fabric at the bottom of the zipper. I also moved one of the turn button fasteners that was inconveniently located right where the boom vang exits. Also added leather for chafe protection where the vang passes through.

First, I trimmed, then sealed, all exposed raw fabric edges with a soldering gun. (Don't have a hot knife.) I then lined the entire inside of the cover with a layer of nylon pack cloth. I bought 2 sq ft of sailmaker's leather and used it to line the forward portion that goes around the mast for chafe protection.

I added a short length of 2" webbing to the inside of the upper flap that goes around the mast. I installed two spur grommets in the end of this flap and stitched/seized a small stainless lacing hook between them. The hook is on the outside of the flap, with the opening facing away from the end. A short loop of shock cord fastens to the grommets with stopper knots keeping it from pulling out. I attached a small handle/shackle pull (knotted leather shoelace) as a handle in the center of the shock cord loop. Fastening the top of the cover to the mast is as easy as grabbing the shoelace handle, pulling the shock cord around the mast, and dropping it in the hook.

I cut the boom end of the cover for a more custom fit and added another turn button fastener at the bottom corner. I cut a slot in the top of the cover to accommodate the topping lift. (I use leach lines instead of lazy jacks to tame the main. These lines slide up and down the topping lift and tend to pull it inboard when the sail is furled. More on this in a later project post!)
The edges of the slot are lined with leather and have Velcro to keep it closed. I also added a carabiner-type sail hank to the one of the top edges. This clips to the "pig tail" that holds the boom up when at anchor. It is not really necessary but does keep the cover nicely stretched out over the boom