Hilbert Curves https://en.wikipedia.org/wiki/Hilbert_curve are funny and convoluted curves that (on their ideal and final configuration) fill the space. Brains and guts and other biological tissues are similar (they tend to maximize surface while using low volume).
Use a 7mm sphere bearing.
A short video showing the ramp at work: https://www.youtube.com/watch?v=HcspDSGJ5UI
This version is the third iteration towards a Hilbert Curve. You can see how each of the 4 quadrants resemble the whole geometry, thus exhibiting fractal behavior.