I once watched a MC shop make a custom cable for me, and it's not rocket science, but there were a few tricks that help make it successful. They had a huge supply of all the cast lead ends that MCs used and since you need spheres, I'd look at sporting goods shops for fishing sinkers that come close to the final size, and sand them down to fit if necessary (wear gloves, don't breath the lead dust, dispose properly, don't let your kids or pets eat it). The trick is to use a counter sink or drill bit to create a 'crater' in the end of the sphere where the cable emerges. Then you splay out or 'peen' the CLEAN cable wires so they fill the crater. I'd use a small vise to hold everything in place for peening and soldering. At this point the MC guys will dip the end into a solder pot full of molten solder, but you can just use a heavy-duty soldering iron or small torch and ACID core solder (NOT rosin core, that's for electronics) and get the solder to flow all into the crater as well as coat the whole lead end and cable strands. Then you trim off the excess wire ends and grind/smooth it out. Done right, it will hold fast for ages.