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Don't worry about the trademark
It's a non-issue, but thanks for pointing it out. I can write about a character going to Disneyland and riding the rides, I can write a story about a character visiting the Playboy Mansion or driving a Ford, etc.
Besides, this story isn't based on the UFC octagon, it's based on an earlier use of the shape as a fighting arnea -- the original "Octagon" was a 1980 movie starring none other than Mister Awesome himself, Chuck Norris.