I have found using a weight plate under the battery to be the best solution. They are usually made in such a way where portions of the plate sit inside the cell slots, and if it's glued or taped to the battery, it keeps them in pretty good.
To me, it seems like the best way to add weight for balancing out the car left to right and still give good handling. Since it adds weight as low as possible, and it's in the same location of the typical nimh cell the car was originally designed around it handles just the same. It may not be all the weight you need to add, but it definitely decreases the amount of ugly lead you have to find room for.
-Korey