I've always felt the best way to do it is to put nothing inside the cover.
Use a dremel tool or sandpaper to scuff the solder point area on both the baseplate and inside of the cover.
Squeeze it on as tight as you can, maybe even clamping it with a piece of cardboard between the clamp and cover....so it doesn't scuff or bend the nickel.
Then, using flux, solder the two spots as good as possible.
Melt some parafin cooking wax (found on baking isle at the grocery store). Let it cool, so it's not overly hot, but still liquid. Submerge the pickup, while hanging onto the lead. Don't let the pickup touch the hot pan bottom......you don't want your bobbins getting soft or melting. After the little bubbles stop coming up, quickly put it on some paper plates. I usually throw it in the freezer for a minute to speed up the hardening. Take it out, wipe off all the excess wax, and install it.
This method will give you a nickel covered pickup that's impervious to high volume and high gain.