If you want to play around first then chop the end of your current grip so that the shaft end is exposed. Get some string a long bolt and some nuts (lots of nuts) you can then thread the nuts onto the bolt to whatever weight you want to add, wrap the string around the bolt and insert into shaft. Keep adding nuts until you find a nice feel, you can even move the weight around by changing the strong length.
Once your happy take out the bolt and weight it, add that weight to your current grip weight and you have an idea of what weight grip you need to find.... Or do it even cheaper and keep the bolt and add loads of hot melt glue and stick it inside the shaft instead, meaning you can add a standard grip of your choice.
