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You could always buy the correct case back opener. Something like this https://www.ebay.com/itm/D-Old-Vint...ck-Opener-Wrench-Accutron-D043f-/200906673334 but it should say on the back which size you want.
Now, the alternate is a rubber ball or something else with grip. If it doesn't move freely... there is a trick, but I'd be castigated for even thinking it, let alone writing it here.
Haha!