Thats a good one. We used to do the same for recoil in our pump shotguns. Drill a hole in the stock about the size of a quarter as ddep as we dare and fill it about 2/3's with 9 shot. Worked pretty well for free.Get some lead birdshot, I used #9 and put it in the handlebar. The bar will resonate no more and is dead.
This worked better for me and really stopped any vibration.
I'd fill it most of the way, but don't pack it full. You want it loose enough that the shot can vibrate a bit and absorb accordingly, but not so loose that turning corners causes the shot to slide back and forth.I've got a 25# bag of #8 shot left over from sporting clays, how full do you fill the bar?