I was working on the rudder today, but ran into this problem. The counterbalance rib doesn't fit between the counterbalance skin quite right. At first I thought it might just be that the ends of the side flanges needed to flex inward a little, so I filed some relief at the ends of the rib to allow some flex, but that didn't seem to help.

Actually you can see that they're almost the exact same size.

The width of the rib is exactly the same as the width of the skin.

I'm gonna have to ask Van's about this.