When I'm on my back, I use towels too. Two small ones folded in half stacked on each other. Then a third folded in four, placed under the neck.

The neat thing about towels is that you can configure the thickness and shape til you find what you need.

On my side, I use a really tall foam memory pillow on top of the towels. All that space between the shoulder and head has to be filled in so your head can be in a neutral position.

I also found that I was unconsciously tensing my neck while laying down. Not sure what that's all about, but I monitor for it and release it.

HTH