Many, many years ago, I bought a cushion cover, just because I liked it. It wasn't a normal size or shape at the time, but I thought, it's commercially made and sold, no problem.
After several searches, I couldn't find a pillow form to fit, so away in the bottom of the cupboard it went. We moved, and again, into the bottom of another cupboard it went. I discovered it on one of my sort outs and thought, it's got to be used or it's got to go.
I decided, it would be quick and easy to make a cushion insert, so in five minutes, with some calico and toy filling, a simple pillow form was made.
Now on my settee, I have another cushion which fits perfectly in the small of my back.
You wouldn't believe how many cushion inserts I've seen now, but hey, this one cost less to make, so I'm happy.