Expanding my comment into an answer.
People will feel more uncomfortable invading someone else's cube if they need to move things around in order to make it usable. This is inherently true of very messy cubes, hence why your tidy, minimalist one gets treated as a guest cube when you are not there.
You have a nametag and a couple of schedules up, so people can already tell it's occupied. You need to set things up to make people feel more awkward and invasive trying to use it.
Messiness is one way to do this, but you clearly prefer things to be extra-tidy. Another way, as a couple of people mentioned, is having an unusual keyboard that's going to be harder for someone used to a standard keyboard to use.
Another possibility is to leave something on your desk and/or chair that people would have to move out of their way. A box to place on the chair, and/or a shawl or jacket draped over the desk. All other things being equal, people will choose a cube that doesn't require moving a jacket to get to the keyboard over one that does. (As the saying goes, you don't need to be faster than the bear, you just need to be faster than the slowest guy. Make it so the "slowest guy" is always a guest-cubicle, rather than yours.)