Mounted the counterweight to the base plate. After testing with 4, 6 and 8 kg ballast, it seemed 6 kg (13 lbs) gives the best balance.
I bought 2 kg (4lbs) lead plates from a local scuba-diving shop:
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http://dykk.no/main.aspx?page=article&artno=MTDA-1
I don't like exposed lead because of the health hazards, so I covered it in liquid plastic and added a shrink wrap tube on top of that. Probably overkill and you can probably use whatever to cover the lead, but I had these things at hand.
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Polytek EasyFlo 60 Liquid Plastic
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PVC Plastic Heat Shrink Tube F/W:95mm
Then I printed a 3D clamp (ABS, 30% infill) that has two very long M6 110mm hex bolts going through it.
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M6 110mm stainless steel hex bolt
Finally adding some common furniture anti-skid rubber pads to both the clamp and the lead to avoid the lead sliding in the clamp.
