This is my favorite site with tutorials. Scroll down to long zippers.
https://www.sailrite.com/How-To-Proj...ps/Tips-Tricks
I made a zippered bag for a boat seat cushion that had a pointed end. Trace around the object. Add about 2 inches for seams and some give in the bag when done. You can make a zipper placket and then insert the placket between the front and back fabrics.
One more
https://www.byannie.com/zippers_videos
You can also use two zippers. Insert so the zipper tabs meet together in the center. The larger zipper opening makes it easier with bulky items.