The 24' H x 48 W x 48 D box sounds great as a stationary target where weight and bulk arent a concern. I'd probably build a little troth in front below the target to catch rubber as it flows out. Maybe a cut channel on each side of the front to slide fresh cardboard or pvc signs in the front to sure up the front form leaking now and then.
Markshere that sounds pretty good but think if you cut the soft but durable sidewalls out of tires and use those in overlaping bands as your front might be more self healing.