I used 3/4" silicon tubing from the hardware store. Cut it to the length of the rubber end that slides in the bar and then make a cut down the length of the tube section. It fills out the gap enough that when tightened down it won't move.
I have used a piece of thin walled pvc pipe split from home depot many times for bar end install shims.
__________________
"Peter, It seems like you have been missing a lot of work recently, well Bob I wouldn't say that I've been missing it"
ENOUGH, go out and ride!