2 sticks of dynomite will get you tha boom-boom
Sorry, it's late and I couldn't resist. Seriously though, if you get a box made or a prefab one, make sure to tie it down using the anchor points for the flat tire. Hatchbacks are a notoriously dangerous place to lay in a heavy box. One hard stop or head-on collision and you might find that box on the dashboard along with your head. Play it safe.