I use bolsters to fill that angle between mattress and headboard which eats pillows: try it to make reading in bed much more comfortable. Closure is a YKK invisible zipper in the seam.
Because the bolster cover squeezes inserts into the desired shape, I suggest filling them with king-size pillows, preferably polyester as the resilience is perfect. Save precious down pillows for sleeping on! You can buy bolsters at a discount as part of the bed makeover package.