I do this more for the feel in my hands than because I want to counter balance a bat, but the added weight up top may help you out. Here's what I do to all my bats.
An extra half layer of binding using extra thick twine (jewellery cotton cord in this case) to the bottom half of the handle. The twine/cord is bought as 2mm on eBay and comes in 10m lengths. I use almost all of a length to do each bat.
Once the additional binding is added and the glue has set it adds roughly 1oz of weight.


The above 2 pictures are on a thicker handle, so I have applied a single chevron over the top. If the handle is thinner I will apply a thicker grip, my preference being a spiral or a ripple.
Here are two bats next to each other, the one with the ripple applied had a thinner handle to begin with, but I doubt you'd be able to tell them apart in a blind test now.