It looks like am impossible task if you see the photo from 2011.
From TEPCO's Photos and Videos Library 4/22/2013:
Reactor 3 Spent Fuel Pool, November 12, 2011:
March 20, 2013:
April 22, 2013. The smaller rectangle to the left of the SFP is where the skimmer surge tank is. The skimmer surge tank is also now covered:
Because of extremely high radiation levels on the operating floor of Reactor 3, all the debris removal has been done by remote-control equipments, with some accidents (and with human workers with tungsten vests on the floor to observe the work). Installation of the cover must have been done remotely also. I'd love to see the video if they have one.