I have solved this challenge a few times using complicated omnidirectional swimmers that can detect both food bits and the substrate wall, but seeing Pixlo's screenshot inspired me to consider simpler designs (though unfortunately I couldn't download their substrate).
Then the workings of evolution on my design inspired me to try another design, which evolved to inspire another! And thus a whole new family of tumbling and spinning flat-earthers came to be.
This substrate has three different designed organisms:
https://www.dropbox.com/s/tkbj5x63xjdg4 ... trate?dl=0
Red was the first concept, a lazy lateral drifter similar to the orange one Pixlo made. This later evolved to have a rapid spinning behavior, which I tried to capitalize on in my design of the yellow trapezoidal model, and the simpler blue version that came next.
Once I had a semi-viable design, I knew I could rely on evolution to optimize and explore alternatives that I would never think of. And it sure did: my lazy lateral drifter became a lean, speedy spinner that could run circles around its ancestor. Something similar occurred with my subsequent yellow trapezoidal design, shedding a phagocyte and becoming the "dizzy shrimp" you see in the evolved substrate below:
https://www.dropbox.com/s/ns0mhlrvvf5g6 ... trate?dl=0
Evolution really favors a spinning motion for these, which makes a lot of sense if you look at how much surface area it can sweep its phagocytes across with a minimum of energy. Their approach to seeking food is haphazard and imprecise, but they more than make up for it by being in a near constant state of spinning, grabbing food in fairly wide radius.
The blue single-flagello species has some of this spinning behavior, but also enters a "resting" state when no food is sensed on one side, letting them conserve energy.
I was a little surprised to learn that wall-detection is not that important in this challenge, if your organism is efficient enough at seeking food. My red and blue spinners do not have wall detection, and I'm not convinced it's doing that much good for the yellow one either.
To others attempting to solve this challenge, I would suggest keeping your organism as simple as possible; start with a low swim speed and careful movements, and don't worry too much about the wall. Get a viable start, then let evolution do the fine-tuning for you. And don't get too dizzy!