The battery and battery straps are usually the #1 part that’s always wrong in a restoration of a BMW R12 motorcycle.  I have taken steps in locating, purchasing and helping with development of the correct BOSCH style battery boxes for the BMW R12. I now have the correct ones to use just like those from the 1938 parts book, 1941 BMW R12 parts book D605/4, and Military manual D605/3. Below are the images out of the parts books. Here you can see 3 examples from BMW documents.
1938 parts book Tafel 11


1941 parts book D605/4


Military operators book D605/3

As you can see, below is the exact same BOSCH battery box as above. The inside of the battery box is empty and you can put a nice solid state battery inside.