My husband made his out of styrofoam circles (cut from big sheets) and a length of PVC pipe. His ended up being pretty heavy partly because of the size (7 feet long), and partly because he coated the styrofoam part with plaster to make it more durable. If you skip that part, and make it smaller, it wouldn't be nearly as heavy (but I'd be concerned about it taking damage, especially in a roller derby environment!).
If you want something really lightweight but that can take a bit of a beating, what about something like those collapsible plastic tunnels? You can get them for cats, for kids, or as laundry baskets. You could cover it in fabric to make it look more like wood, or metal, or whatever look you're going for. You could probably still go with PVC for the handle, or a broomstick, or whatever works for you as far as size and weight goes.
Good luck with your build!