C.O.G (Center of Gravity) location is one of the most crucial parameter provided by the model’s manufacturer. The proper C.O.G location enables easy and stable flight. However, some pilot prefer different C.O.G location; deviating from the factory setting in order to obtain unique flight characteristics. They can be nose heavy or tail heavy. In most cases, battery mount location is adjusted to obtain the desired C.O.G.
There are some cases where the desired C.O.G is failed to be achieved, for example when using a much larger and heavier battery or smaller and lighter battery. This might also occur when using FPV models without mounting any FPV gears (just for flying the model). Additional weights called counterweights are used to solve this issue. There are many ways to find additional weight to be used as a counterweight (coins, stones, etc). Solid lead sinkers for casting/fishing are another option.
They come in a variety of sizes and weights. You might directly use and load them to your model, or you might want to merge a couple of them into a single shape.
Firstly, the required weight is to be measured or you could try loading some of the lead sinkers to the model until the desired C.O.G location is achieved.
Secondly, construct the mold / die using wooden materials to a desired shape. Cover the internal part of the mold / die with thin aluminium sheet.
Finally, the lead sinkers are to be heated until they reach melting point and formed into liquid state. Immediately pour in the liquid lead to the mold / die.
Notes : Consult a professional for heating and melting process as the gas fumes from lead is extremely hazardous to human health.
“Don’t confuse symmetry with balance.” ― Tom Robbins, Even Cowgirls Get the Blues