when upgrading my starter wires I managed to snap the +v connection bolt!!!

Badly rusted nut that despite heat, cutting the nut etc it still snapped the shaft and as it turned wrecked all the insulation.
a tip, put a hole in the black rubber cover so when water gets in it can drip out and prevent the nut rusting to the shaft. He also said paint over the connection to seal out water.
Now I have a new bush and shaft, but the insulation to prevent it earthing to the starter body is a problem. I will have to make some but I ask why not just solder a suitable thick wire to the new bush wire and then join the starter wire to it out side the casing? then seal up the hole to stop water etc.
So to clarify, an insulated wire will come out of the starter body and then is connected [nut & bolt] to the starter wire from the solenoid.
The advantage is that I would not have to make up some insulation solution for the +v bolt
The auto elec was not happy with that idea and said the +v bolt had to be tightly fixed to the starter body, but would not say why my idea wouldn't work.