You are going to have to take off your throttle tube and slice a little bit of it off so the opening is the same as your handle bars. It won't affect anything. I used a pocket knife with a serrated blade to do mine when I installed hand guards (same principle).
I know that when I installed my Dad's hand guards on his aluminum bars I had to use the rubber inserts, but when I did mine on my steel bars no inserts were necessary. I'm assuming you're missing some sore of rubber insert. I should be a round tube looking thing.
Don't be afraid to slice your throttle tube. You don't need to take off very much...just enough to make the opening match the ID of your bars.