Available on Ebay
I looked into creating my own circuit for this, and decided that I simply didn't want to do it myself, so I looked around for options and found a place in China who sells just what I wanted both on Amazon, and on Ebay.
I decided to give them a try about 2 weeks ago, and I ordered two of them. They cost $33 USD each. Kinda pricey, for what it is, but these devices look like they'll fit nicely, and then also include a time range so if there's just a brief voltage change a reaction can be delayed from 1-15 seconds. So for example if set to 15 seconds, and there's a voltage change for 3 seconds but then it goes back into the OK range then nothing happens. Like a buffer.
So for my system I'm planning to try to tie this into the soft-start relay portion. Basically if voltage isn't between 46 and 56 vdc I want the whole system to shut down (at least for now) and that'll happen by wiring this voltage monitor's relay terminals in to the primary relays somehow. (yet to be determined)
Here's a little video demo of this device functioning.