The further from earth you move the thing...
...the smaller it can be. Move it out to Jupiter and it can be the size of a coffee can (no I haven't done the math, but at interstellar distances, these are point-sources that emit the hard radiation).
However, the further from earth it is, the harder it is to do stationkeeping, so it's shadow is on earth for the span of the event.
Anyone know how long we would need the shield?