I imagine you mean to make it like an envelope, so it just slips over the back, I would make it long enough to fit over the back rest of the chair in one piece, stitch up the sides and neated off the bottom, I would have thought if it was snug enough it wouldn't need securing, her weight on it should keep it in place, and if it was me I would use batting, as I think it would help absorb her sweating, I would also make two so one can be in the wash!!! Hope this helps