I haven’t found the need to do my 800 GS as those bars are pretty calm at hiway speeds but I have aftermarket handguards with the non oem handguard end weights.
I did put birdshot in my KLR bars and that made a noticeable difference in the engine buzz range. I foam filled the bar center as weight in that area doesn’t do much except add weight.
I used #8 lead birdshot in the bar outer ends then silicone sealed the bar outer ends. I don’t suggest BB’s as those are copper coated steel and will rust into a big glob after a while once water gets to them. You also want the added weight of the lead as well as the more surface area on the smaller birdshot.