Short of someone shipping you that one part, a suggestion:
Cut a piece of the thinnest sheet metal you have around - an empty beer can's sidewall might do nicely.

Cut a circle just a few mm smaller than the inside of the case back(so it won't float too much). Then make a few short slits inward from the edge(3-5mm...1/4") and bend up an a corner at each one. You might want to do one in the middle to keep the metal against the batteries

Basically those little "ears" will apply a little spring pressure to keep the metal against the batteries AND complete the circuit to the case and onward to the buttons. Your particular module take a clip that looks like a fancy bow-tie with two "ears" protruding from the one side for snapping into those square holes between the batteries.

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