Just a few thoughts, because I like to over-complicate things.
1. Is there a way to make it so that the cup holders fold down out of the way, so that a passenger can sit without having to remove the holder, but have it ready at a moment's notice? It would be easier to flip up into place while driving, rather than finding pins while trying to stay in your own lane!
2. My only worry about the pins is the really easy ability to lose them! Anyway to make them not come out, but still somehow lock/unlock? Maybe spin to the side exposing the "hole", allowing you to slide the cup holder out?
3. It seems like you want to make different "containers" for the bracket besides cup holders. What else do you want to make? I know the cup holder is enough to begin, but you're going to want to make sure that any other "container" works with the bracket. Maybe one for garbage, one that's a half container and half cell phone holder?
4. Is there a way to put a square notch on opposite sides of one of the cup holders to be able to hold a cell phone, yet still make the hole be able to hold a cup (obviously the person would have to pick one or the other). This way, you wouldn't need to have extra "container".
I have a '99, but would love to see what your final product looks like! If you are still looking for other ideas for that list of other possible projects you could have done for class, you could mock something up for holding the rear headrests. I know some people here have done their own thing for them, but you might have an idea for the list.
Good luck!