I literally stole this from @iacas (thanks, man). It's a posture board helps you keep the center of your hip joints from straying too far back towards your heels too much (which I tend to do). You can use it for early extension too, I guess. And, you can hit balls with it. All of the elements of this extremely high-tech training aid are easily obtainable at your local hardware store. I'm not even that handy, and I managed to build it: