Battery holder fabbed up and sitting on crossmember. This took some thinking. I just had two M6 bolt holes on the "frame rail" where I wanted to mount it. There are several more large holes in the rail in that area. I got the idea to use a well-nut in the holes. Since it was 20mm hole there weren't any well-nuts big enough. A bit more thought and I realized expanding boat bung-plugs were exactly the same thing as a well-nut and available in 20mm.