I very much prefer the nature instead of dealing with questionable restrooms or odd devices. Luckily we have plenty of ditches and trees around here so "privacy" is not really a problem (and if the guys see my head/helmet poking out so what?).
If you are in a less wooded area and you can't find a rock just take your jacket and hang it from the bike and voila you have your privacy screen
(watch out for exhaust so you don't burn a hole in your jacket) - trust me the guys are way less offended then you think.