It's easy to fix a battery compartment where the spring tab has broken off. Get some 0.008" thick phosphor bronze.
Cut a piece to about the size shown. Note the two clipped corners.
Using a center punch or even a nail, place the piece on a soft pine board and gently tap a depression in the center.
Place the piece in the battery compartment with the clipped corners up and the protruding side of the dimple toward the battery. The clipped corners let the battery cover slide into place. The battery will wedge the phosphor bronze piece between it and the Cab-1's conductor and it will work just fine. I've done several of these.