Cutting yourself a 12" long section of pool noodle might work well. They come in several diameters and can be purchased with or without a hole down the center. Last time I was at my local dollar store they had pool noodles for sale, so it could be a very cheap solution if it works well for your situation.
The cheaper medium grey foam water pipe inulation noodles might work as well (the slightly more expensive black foam type would squish flat under your head's weight so don't use one of those). It's the same thing as a pool noodle except it has the slit down the length for wrapping it around a pipe and sticking it to itself.
Another option that popped into my head was a small squishy boat fender.
Lastly, a squishy foam kneepad like the ones they sell for gardeners. The ones that feel like a dense memory foam. The nicer ones are often coated with a thin vinyl dip that makes them very easy to keep clean.