Rigged this up just to try steering. It was a hoot. The bars remain fixed and I steer by moving the phone.
Three rubber bands and a zip tie. Double a band around the top end of the phone. Attach side bands. Use a zip tie to take up slack around the phone and center it. The back end of the phone simply rests on the top of the stem. Steered with my figertips pushing/pulling the rubber bands. Or thumbs pushing left/right on back end of phone. (hint: use countersteering)
As you probably know: To get on the mtb track, select “Muir and the Mountain” route in Watopia. Ride south ~2.2 miles and select the grey ? right turn when it pops up. After a minute or two, it will ask if you want to try steering.