Ceiling Fan Cathedral Ceiling Help! I Need To Install A Dance Pole To A Slanted Ceiling. How?

Help! I need to install a dance pole to a slanted ceiling. How? - ceiling fan cathedral ceiling

I bought a pole dance and want to install, but I have a slanted ceiling. The base of the mounting bracket is 15 inches around and needs a flat surface. I would like to build a kind of box or something, to give a flat surface. I've also searched online for a card with a high ceiling, but not flat, but in fact, like ceiling fans. Has anyone received any suggestions to this bar? Thanks for the help.