The purpose of "Direction Changers" is to have the dominoes turn around by 180° on very tiny space. Also, they save the 20 dominoes you would otherwise need for such a turn.
A Direction Changer is quite a simple tool. It consists of just three parts: The base that is about 3.2x1.6x0.4 inches big, the nail (1.7 inches) and the swinging block of 4.8x0.8x04 inches. Just buy plain wood and cut it to these sizes with a saw.
Once that is done, you need to tag the middle of the base. That can be quite a boring task if you're doing what I did - not just produce 1 DC, but 96 of them.
The next step is to drill a hole into the swinging part for the nail to be put into. Then all that's left is assembling those parts - hit the nail through the base and put on the swinging block. That's it.
BEWARE: The use of DC's can cause injuries. Especially if the swinging block sits on the nail only loosely. I haven't heard of a case of a domino-builder accidentally stepping on such a nail, but I still recommend caution. I can't assume any liability for your safety.