I really wish I could find the pic's, but I will try to talk you through this...
get some 1x2 scrap 2 pc about 2 feet is great, then you take cardboard and staple it on both sides to make a sandwich type box, then you take another pc of cardboard and glue a standard police human type target to it, Now slip it into the "sandwich" (make sure it is really able to slide up and down, now lift it up as far as you want to expose (for head shots only lift it so the head shows, or if you want full body shots lift it as far as it will go and still be stable) then on the back side you duct tape a small water balloon to center mass and where it holds the target from slipping onto the sandwich, when you hot a good center mass shot it pops the balloon and your target drops, the smaller the balloon the harder it is to drop the target.