If you just want a panel mount voltmeter, they are dirt cheap. You can get them that just require a round hole (hole saw it) and pop it in. Once you learn how to interpret what you are seein gyou will be able to manage your batteries quite nicely. You only need resolution to a tenth of a volt but accuracy is paramount so validate whatever you buy against a meter you know and trust. Hook up as lose to the batteries as possible and be sure to have good connections.
There is also this which has the round meter in it which could be useful:
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