Adjust the top spring if you can, needle nose pliers and a bit of a twist pull without accidentally pulling the spring itself OUT oops.
prefer to not use magnets, or "partly loose metalic objects" they will work fine, then remember the time something got hard dropped? and gravity sent things into other places
then when they get in the other places, they grind away for a while then pass through insulation, and then you have a very strong dead short, and major unhappiness. a hunk of metal crossing from the top post to the edge of the can makes the best short you can get
you see the top shoulder lift up there in the mag, it might have an original purpose of not connecting when batteries are shoved in with the incorrect polarity. in less than 10 minutes you can extract the switch module, dremmel off the ring a bit, and defeat the polarity reversal mechanism. then anything you shove up there will connect to the small spring even if it is reversed.
If your really fast and good with a soldering gun (practice somewhere else) you can put a dot of solder on the battery button without destroying the cell or the seals and devices under the button, but you would have to be fast and good, and then that still leaves the problem of BUTTING into the bottom of any cell it is combined with. some of the reasons they can BE flat topped, is so the nipple doesnt CRUSH into the bottom of the next cell.
so your bestest choices given the choices is to further modify the mag itself, beings at this point I doubt it will ever be using normal cells again anyways.