I thought about doing this to begin with, but instead I've used thin roofing lead in the battery slots, cut into the right sized little rectangles so they fit in closely, they work out as 16-19g blocks, and you can get up to 10 blocks in in total if you really wanted (in the central slots you can fit 2 layers).
Battery holds them all in place nicely, or use small sponge pads to hold loose ones/single layer ones in when needed.
Works nicely for me as it means I can take them in and out and move them around, 1 by 1 to get the right balance adjustment on the car...really quick and easy, and no messing around with hot/molten lead!
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