I use the tag end of my tiedown strap, its long enough for two wraps and two half hitches around the tiller. Works great and I don't have any extra rope, cord or bungee all three of which I have used.
I just use a low-tension bungee. You don't need a lot of force to hold it. Actually, I don't even bother with it half the time. No biggie if the motor swings around a little as long as the "co-pilot" screw is adjusted properly. If it's too loose, the motor can bang around from side-to-side. I don't tilt the motor during transit, which makes a big difference.