If we settle at about double the preindustrial CO2 level, we would need to block about 8% of the solar radiation [0]. You could do that eg with floating structures of ultra-thin translucent sheets that reduce light intensity in their shadow. To maximize effectiveness you can put them in a sun-synchronous orbit, so it casts it shadow always at high noon [1].

However this solves only the warming itself, it does nothing about things like ocean acidification.

0: https://www.newscientist.com/article/dn11993-solar-shield-co...

1: https://en.wikipedia.org/wiki/Sun-synchronous_orbit