If your priority is a private secure place to lie down in , and you would not be wanting to use it in wet weather, you could have six foot fence panels enclosing a nice space.Or you could even have a big enough fenced space for a wee two man tent, and a Keter box for your spade , fertiliser bottle, bag of compost, and fork. Fence panels although too big for your car, are normally put up by the fence people who sell them, as they need strong windproof posts.One of the panels can incorporate your padlocked gate.
You could do it cheaper by having screening which comes in rolls, instead of wooden panes. The can screening is rather nice, and you can also get willow screening. You do however need an able person to put in the posts, which in the case of screening should be wooden posts.