The battery could definitely be a backpack design, though. Make the device flat-bottomed with rubber feet, so it sits nice on a table, but also have slide-lock notches to slap on a battery. (I'm thinking 3.5" drive but 2/3 the thickness for the bare device, and that much more for battery.)

If you do it right, you could slap a HDD on instead of the battery pack, and power the whole mess via wall-wart. (And if you do it really right, you could slap on both, and get limited shock-resistance and battery-life, but still have mobility and cheap storage.)