Project finished.
Today I got the batteries snugged down & wedged in place, wire connections cleaned up (taped), the mower "hood" back on, and the digital voltage gauge installed.
Ran into a slight snag with the gauge: it's hard -- OK, practically impossible -- to read an LED gauge in direct outside light! Didn't consider that when I bought it.
So I couldn't mount it directly on the mower's motor cover, like I had been planning.
The workaround for the problem: I made a deep "visor" for it, from a black plastic bottle, by cutting the bottom out and cutting the neck open enough to mount (tape) the gauge inside at that end.
Looks like this now: