I did this so I could keep the battery inside the back of the truck. I really did not like the other mounting options. I don’t think AGM battery s need to be vented. But If there is a over charge or problem they will vent. So its better to be safe then sorry.
Now this is a great idea and so simple and cheap too. Just use a a dixie cup and you will know if the power went out long enough to defrost your stuff.