Go for a flail. Is it ineffective? Compared to conventional spinners, yes. But it's something a bit different and I'm sure it'd be fun to use. At my very first event, there was a robot called Pannibalism that had heavy flail weighs on ropes, but they were also elasticated so that when the robot wasn't spinning on the spot, the weights were retracted, reducing the risk of getting caught on your own weapon.