If you are still using ISBoxer, the way I do that is with a multi-step mapped key with delays built in so it only casts it 2-3x per rotation. It's not exact but it works pretty well. I'm at work at the moment but if I can remember later and nobody else gets you a better answer, I can post my config for reference. It's a few months out of date but it should be substantially functional even in the latest patch.