You want the slit to be slightly taller than your clamp and your HIC shim to be super tall, but with a slit the same size as the one on your bars.
That way there's no pinch point on the fork and/or the top of your HIC rig on the inside of your bars.
Just take a set of Ultra Pro bars, cut a three inch section off, measure and cut the slit, make sure it sits straight on top of your headset, and use that for your HIC shim. That's what I've been running for months and it doesn't come loose and is very strong.