From the Blog of Meridith Richter:
I set out to find the best way to achieve the best layout for the switch so that the user interaction would be as simple as possible.
I tried out three different sizes of tube as well as three different sizes of ball bearings, settling on tubing with 5/16 in. inside diameter and the smallest ball bearing size I had. The medium size fit nicely as well, but with my constructed pins out of brass nails, the smaller balls made a better connection as they sat firmly between the pins rather then resting on their tops.
I decided on using two balls of the same size at once in order to achieve a better connection thanks to the added weight pressing down on the first ball.
I ended up with a rounded “L” shape as depicted below, which remedied the problem of someone lying on their right side as well as their left (for the most part).