Here you go !.I need to get a better look at it may be some can post a pic of the factory connector?
Yeah maybe ??????.Looking at that picture I would say it's easier to just bolt on the positive side no cutting need
Ps my garage is not normal in a mess
I have M4 to M12 not a problemit's in the same condition as mine mate !.
Pure guess, but that larger nut looks like 13mm spanner size to me ??.
Would need a bigger wing nut, but it would need a metric thread also ????.
A friend with a lathe is what you need.
Short length of HEX bar, about twice the length of the original nut ( to clear the top of the battery case ) then machine a thread on the inside.
Weld a short section across the top to form a "T" Bar nut.
Your done.
No spanner needed !