I was thinking having the projectiles being aerodynamic may increase decay time, then realized you can have the launch vehicle use solar power to fire the metal spheres using a railgun mechanism. This way, you can get vastly larger areas of denial by firing at an angle perpendicular to that of travel. You can have an every-shifting debris field if you fire 2 ball-bearings (port and starboard) every 10-50 meters to make that entire altitude unusable[1] - not just a specific orbit.

Edit: 1. The perigee and apogee will differ for projectiles fired from port or starboard based on launch vehicle inclination and the resulting relative speeds to earth. It would be a nightmare to track and avoid the resulting mess.