Of course if you are going to use a foil all the way then you need only one solder pojnt, to the original battery contacts. I know conductive glues exist but I di not know how good they are. I would be a bit concerned about how it performs under heavy load. The only way to find out is to try. Please report your results.

I suggest you bend the ends as per mrsellout's drawing. You want to attach the conductive side to the battery terminals, not the sticky side.