It's made from a scrap of plywood, a small piece of 1X6 left over from a door frame I built, eight wood screws, one 1/4-20 thumb screw with a wing nut, and a ball head adapter that I've used for a lot of different things over the 30 or so years I've had it. Oh, yes, and a cardboard box as a light shield.
I used my Android phone as the text display, with a program called Android Prompter, which was, of course, free. The unit I built is large enough for a tablet as well. There's software available for Android tablets, iPhone, and iPad, though I don't know of any free software for devices that don't use the Google Play app store. Feel free to point me to any that you know of, especially if it has a good mirror feature, which is essential for this kind of teleprompter.
Since none of the tablets in my household can use Android Prompter, I have an alternative method for getting larger text; I happen to own a 7-inch TFT television, and my phone can output NTSC video with the right cable (which also happen to have from another device—I never throw out cables). This method would also allow an operator to control the text speed for me, which might be very helpful.
Thinking of that also made me think of another application for this cobbled-together device. You know how people who are taking video of themselves, with a Web cam, for example, are either looking somewhere other than the camera lens, or can't get themselves placed properly in the frame? Well, if you output from your camera to a monitor, and placed the monitor in the teleprompter so that the top of the monitor faces away from the camera, you could see yourself in the glass while looking straight into the camera. Not only that, but the image in the glass would turn the same way you do, making it easier to make the little corrections in position to keep yourself properly framed.
That's another trick I'm going to try, when I have a video to shoot that I've memorized the lines for, or where I'm just talking off the top of my head.