It had to happen... a sharp-eyed Rough Linen fan spotted my own Orkney linen bolsters and just had to have some.
I use bolsters to fill the angle between mattress and headboard. Closure is YKK invisible zipper.
Because the bolster cover squeezes inserts into the desired shape, I suggest filling them with king bed-pillow inserts - easy to buy, and far more comfortable than foam shapes.