This was my first attempt at a homemade ballast - a 6" PVC pipe section, filled with transformer iron (offcuts from a transformer manufacturer). It is wound with many turns of 16mm2 wire, though I can't remember how many as I built this years ago. It worked pretty well, but could not limit the current down as far as I wanted to go, hence the ballast below:
Ahh, this is more like it. This is wound on an old 10kVA pole pig core. The current is controlled by varying the air gap with plastic shims. Quite accurate current control is possible from 20 amps to whatever the wire will carry. It has under 100 turns of 16mm2 wire, with a resistance well below 1 ohm as I recall. It really is quite efficient when used with the big 12" pig powered coil.