Forend T-Slot Weight – Kinetic Research Group
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^^ Official KRG product. Bolt-in, lead free 0.75lb, just $70...
Alternatively, if you have a mill, or are handy with a set of chisels or a Dremel, you can cut a pocket in wood in the shape you want, and fill it with molten lead. It should be safe enough from children tucked away in the fore-end. If you're still concerned about the lead, you can get a pound of bismuth from Rotometals and it'll be "non toxic," It's just less dense than lead.
Example. Not a KRG, but you get the point: