To accomodate the narrow center of the bottom seat, we used two methods to shape the fit and add holes for the female rubber mounts.
We were unable to position the strap shown in a satisfactory route around the rubber mount. To cut the hole in the fabric, we heated a piece of 1" metal conduit with a bernzomatic torch and melted the hole that stretched over the rubber mount. Time will tell how smart this was.
Then, to help the straight cover fit a seat with indentations in the front and rear edges of the seat, we poked holes in the fabric and threaded small nylon rope in an "X" configuration to minimize wrinkles as seen from the top and to keep the front edge seam as straight as we could. This "X" pattern also helped minimize the extra material touching the safety belt attachments.
HTH