When I was a teenager I lived out in Wales in the country in an old farmhouse in the middle of nowhere. This provided plenty of opportunities for masturbating out of doors. There was never any real risk of getting caught, I always kept an ear out for people approaching and chose secluded locations. It was, in fact, more private than doing it in bed (since I shared a room) or in the bathroom (no lock on the door). Though, of course, I did (and still do) masturbate in bed every night.
One of the places I used often was behind a large fallen tree in the corner of a field, sheltered by nearby hedges. I'd sit down leaning against the tree, unzip my trousers and pull out my (usually at this point, half-hard) penis and masturbate. When climaxing I'd turn awkwardly to the side and shoot onto the ground so I didn't get come over my clothes.
My favourite place was a secluded clearing in the middle of a dense thicket of hazel. I'd lie down on my side on the soft grass, push my trousers and underwear part way down my hips and pull my tee shirt and jumper up my body (I never undressed any more than this in case I heard someone approaching). I'd then masturbate (slowly at first and taking my time) until I came all over the grass and my hand.
When I said earlier that I was careful about the locations there was one time when I got careless. I was out riding my mountain bike to some local woods that had some decent trails. I was really horny for some reason and I became semi-hard. The feel of the bike saddle between my legs and pressing against my crotch made matters worse; I had to masturbate and I had to do it now. I'd reached the woods at this point and stopped at the side of one of the tracks, got off my bike and went a few metres into the woods and stood by a tree. I didn't want to go further in and leave my bike behind and at the time I thought it would look like someone taking a leak, with hindsight, it would have been obvious what I was doing if someone saw me (but no one did). I was also too aroused to really care (though I still kept an ear out). I yanked my shorts over my erection and masturbated furiously. I quickly reached orgasm and spurted over the tree in front of me.