Send a factory to a carbonaceous asteroid that builds a factory that builds a larger factory that makes plastic films, coats them with metal and has them fly back to the Earth-Sun L1 point as solar sails.

I can't decide if it makes sense to deal with the problems of sending people that far or deal with the problems of doing it all through remote control with some people living at some place like Earth-Moon L1 who have a replica of the system that they can do experiments on.