My first reaction would be to piece the top, but make about half the side segments from a fabric with some give, such as a high quality fleece (not a minkee type), stretch denim, indestructible 1970's double knit etc. This will make it much easier to fit snugly and take it off and on. The zipper should go in the seam between the side & the bottom. Make it as long as possible. If you just put it in the bottom, even going edge to edge, it will not be a large enough opening to get the cushion form in and out. I would make the bottom canvas or denim.
Last edited by PaperPrincess; 01-17-2018 at 09:43 AM.