Here is something that may give you ideas, it's pre-made to fit a Group 24 battery.. If you use lead acid storage batteries they MUST be vented so that you don't have toxic or explosive gas build up and/or enter your living space.
Amazon.com: MTS Company 200275 Vented Battery Box: Automotive Note I found this from other suppliers for MUCH less than the price shown on the link, I paid less than $ 25 including shipping.
You could make something like this any shape you need, just have an air intake in or near the bottom, and an air exhaust at least 5" above the top of the batteries in the box.
On my Vista the lower back wall part of the basement compartments don't go all the way in to the F53 frame rails so I was able to vent thru the back wall of the basement compartments when I added my 3rd deep cycle 12 volt marine battery, using this box.
Note a rough rule of thumb is aprox. 100 Amp-Hour of storage battery for each 100 Watts of Solar Panel.
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