So I've come across a great deal on Amazon - 4 soft and light cushions with zippers which would fill any cover of the same size or even larger. They are allergen-proof since they are filled with 3D PP cotton.
Whether you want to have support for your back, neck or simply to have a snooze on your sofa, these cushions are great for any purpose.
I was impressed with a number of 5-star reviews for this product and taken by the idea of using the cushions as fillers for all sorts of exotic covers I've found on Amazon. Look out for my post where I'll share what I've chosen for my covers! ;-)