5S, Poka-Yoke and Visual Controls

I recently attended a 5S audit that takes a much more holistic approach, namely it includes the 7 wastes, TPM wastes, mistake proofing, etc. Upon my return back to the office, I encountered a problem in my own genba. When traveling, I frequently run out of juice for my laptop. It takes me awhile to figure it out, but it is because I plug my chargers in backwards and don't realize it. I think the pictures tell the story:






Here is the receptacle. Notice the small key on the right side of the oval.

Here is the plug. Notice the keyway on the side of the plug.

The designers made a good attempt at mistake proofing this plug, but the key is really small and the material is fairly pliable under force. The result is that you CAN plug it in backwards and not realize you have done it wrong. You won't get power, but it looks correct. A blue light comes on, but not always if the power receptacle in the wall doesn't provide power, which surprisingly happens frequently in hotels and offices. (Breakers turned off for maintenance, or in need of repair for example.) So whether if wrong or right, a lack of blue light doesn't guarantee I've done my part correctly.

I need a visual control to tell me if the standard is met, in order to avoid mistakes or failure. Here is my visual control for this standard: