I recently acquired the Decorbox The Union Jack British Flag Cotton Linen Square Decorative Throw Pillow Case Cushion Cover 18"X18 18X18 Inch (link) and I find it to be acceptable. It's made of strong linen material, with an invisible zipper, measures 18 inches square (45 x 45 cm). Unfortunately insert are not included but overall this pillow cover serves its purpose well enough for me to recommend it.