If you use a large flap over your zipper, it can cover the zipper also make it low on the pillow I have several like this looks very nice to.
Another idea would be not to have either a zipper or cams. Instead split the back part making one bigger (so they overlap) and leave a opening. I am not sure if I explained this well enough.
I think this style is called an envelope cushion/pillow.
This is what I would go with and place the stuffing in a separate enclosed pillow so in can be inserted or removed as needed.
If you make a cover with a zipper you can always stitch the pull tab down with a few stitches that can be removed as required for washing
Interesting question... If you want to be able to wash the pillow form without the pillow cover, I might use Kam Snaps on the Pillow cover...