I use a combination of my first suggestion and what Gary has suggested. I have two 7(1/2?) lb weights drilled out and two pieces of 75mm mild steel bar (50mm thick) with bolts taped into them. They are very handy. With a 21lb (Losmandy), 3x7lb weights and these two I can mix and match as needed. Mind you this is enough for two mounts most of the time.
Oh and a 7lb sledge hammer head that can be bolted to the bottom of the shaft if I need it
