The folks working on the L1 case (e.g. Robert Kennedy and his students in TN) estimate the shade would have to be approximately the size of Texas (about 700,000 km2, much smaller than you propose) and could be erected within 50 years.

Their plan is to make a lot of small devices rather than one large device. Just talking to the structure(s) is complex as there will be a huge noisy EM source right behind it.