Four wheel drive indicator?

[AirSpeed]

It seems that every time my wife rides in my 72 4X4 she insists on sitting right next to me and almost always seems to knock it into 4 hi with her long legs. I never seem to notice it until the fuel gauge drops faster than usual. I'm wanting to add some kind of indicator to let me know when it's in four wheel drive. I'm thinking of putting something in the empty hole next the oil pressure gauge. Any of you done this or seen it done? I have a couple ideas but wouldn't mind seeing what others have done. Thanks!
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