On occasion we are given the unhappy task of informing our customers that the zipper on the short end of the 12 foot long cover isn't anywhere near large enough to let us insert a cushion. When you have your cover made for that long window seat, make sure that you have the zipper installed along the entire width of the cushion. Zipper is the least expensive component in your project so be generous!
(actual cover with too short zipper)