"Original" generally means that you did the work to make your own representation of the concept. "Derivative" means that you based your work on someone else's work (for example, most of those Middle Earth maps are in some way or other derivative works based on subject matter and often layout, but are generally considered "original"). "Plagiarism" is where you would be copying everything about the composition and notations on the maps, doing nothing of your own except some stroking.
From what you said here, I would think that your hexkit map would definitely qualify as "original". It shouldn't qualify as infringing in any sense that would cause anyone particular distress. Post it! If I understand US copyright law (I am not a lawyer), the worst that can happen is that the copyright owner requests that the site pull it down and it will be pulled down. At the very least you'll get feedback about what you did. Sometimes, you'll get comments about the original source material.