Walk onstage carrying an ordinary briefcase and set it down on the table. When you open it up, remove a basketball or other large item that cannot possibly fit in the briefcase.
After the production the case can be shown all around.
Show BOTH the inside and outside of the briefcase at the end of the effect.
Carry your important documents in the case or even other "packs flat"effects.
Top quality briefcase and professional construction make this a winner!.