I wouldn't modify the plastic box, as the original military style batteries are available at your popular professional parts stores (ie; o'reilly's, napa, carquest, not your Walmart of parts stores, ie; auto zone, pep boys, etc.). The original style batteries are not much more expensive (10-15 dollar a piece), but have more power and more reserve, even if the spec sheets don't say so. Group 31 batteries are good batteries, but the originals have lasted a year or two, sometimes up to four, longer then the group 31. This has been my experience at my vfd.