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My EV has eight batteries tucked under the rear seat. Three are completely accessible; three are accessible enough to put your hand on the terminal and wiggle it. That leaves two stuffed so far under the sheet metal that you can't even see the regulator lights blink without a mirror.
Inaccessible batteries are a bad idea in an EV. At the very least, you should be checking the connections periodically to make sure they're still tight. And placing your hand on a terminal to check for heat can be quite revealing. So we decided to cut access holes.
Frankly, it's embarrasing how long it took for us to do this. We planned, replanned, gathered materials, and tossed our plans out the windows in the face of physical evidence. We drilled pilot holes, moved batteries around, put in partial packs for fit, and tossed the new plans, too. Finally we gave up: we can't cut holes for all four terminals of these batteries, unless we cut the entire rear sheet metal out. (I believe the racers call this "tubbing" the car.) Then we'd need to replace structural members, make a place for seat belts and hinges to mount, and make a stop for the seat to rest on.
Once we had decided what to do, cutting the holes only took 10 minutes or so. (Although that's an eternity in the hatch of a wagon in Florida summer.) And they're pretty rough holes, too. As Nathan says: "The nicest thing you can say about these holes is that they'll be covered by the carpet."