"Jeez, I don't know why everybody is making such a big deal out of this with all the doom and gloom and stupid idea comments.
It's a pretty simple, easy, safe fix.
Don't use a mold and pour anything molten into that form- forget the form.
Screw a bolt into the terminal so you have something to work around.
Use a soldering gun and a roll of solder. Fill the void areas with the solder a little bit at a time, leave it to cool at the first sign of any of the plastic starting to melt.
This is a 10 minute and 1' of solder repair.
I bought an old Shovel low rider some years ago that had a brand new battery but the terminal was torn right off. That one took me 1/2 an hour and a few feet of solder.
The solder is easily sculpted with a soldering gun and the whole thing can be made to look almost like new. "

That's the first part of the plan. I've abandoned the lead idea


"Try a fix and if it works cool - if not, I would walk away and do plan B. "

And that's part 2. Plan B is a new battery

I'll let you know how it turns out.