I made this one completely from scratch in Audacity. I generated the tones, spliced them together, and also made Shepard tones from scratch.
I haven’t been making these in a while. Here’s one I created…I don’t know when, a while back. Originally known as Frequency Out because it was a mixed down backtrack of a larger project that I never got around to completing. FreqOut made me snigger, so that’s what I went with.
Feel free to use it in your project if it fits, or just listen to it in the background while you paint, or meditate or write, or whatever. It should stay out of your way enough to concentrate, while challenging you enough that you don’t fall asleep.