Bondo alone ain't gonna save you if the bottom of your battery box is rusted through. If it isn't completely rusted through, I would wire brush the heck out of it, and apply several coats of something like Rustoleum Rusty Metal Primer, and keep the coating up. If its rusted through on the bottom, I would get some heavy gauge sheet metal and bend it up into the shape you need, and weld, bolt, or rivet it into place. As a witness to and a veteran of a few temporary rust repairs to 1960s and 1970s rust bucket cars when I was your age, I know that Bondo will crack and bubble up eventually, usually sooner rather than later, especially if it gets any strain on it. Think about that battery as a 25 pound weight just bouncing up and down on your bondo work, and what it will do over time.