Believe it or not, I found my 14mm driver (and two others - don't remember the sizes) at the local Autozone... all three come in a pack that's pretty cheap.
My fill plug was very tight.... maybe 125 lb-ft or so... needed a long extension and a long breaker bar to get it loose...
At least in my case, a homemade tool wouldn't have cut it.
Good luck.

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KenP

1996 S500 W140 Coupe