The beauty of this idea is that it is a passive signal requiring no broadcast energy or targeting. And, it is detectable over a sizeable volume of the Galaxy. Nor does the billboard require any long-term commitment for maintenance by the host civilization. Set it up and forget it. If the home planet's space budget dries up, so what? In principle a space shade could be designed to last for millennia and even outlast the lifespan of its civilization.
An alternative approach proposed by Arnold is to have a freight train of objects successively transit the star sequentially, and in a distinguishable manner. "At each period, we would observe a series of transits whose number and timing would claim its artificial nature and a will to communicate," he wrote. For example the sequential transit could communicate a series of prime numbers or a binary code.
The billboard would need to be placed close to the star, far inside its habitable zone, so that successive transit could be seen every few days.