Al, I'm not sure you want to close that cement structure off as it could cause problems if there is a large rain event. With that said, I'd be inclined to make something that would prevent a person or small boat from being sucked down when it is running. Given the 10'x12' dimension, I would weld up a frame of 2"x4" tubing and then run 2"x4" tubing on 14" centers across the frame. That would allow 10" spacing to prevent trash buildup during an overflow. Attach the frame to the concrete with concrete bolts. That's one way to do it. Others may have a better or cheaper way to fix it. I have personally seen some serious dam damage due to trash clogging an overflow so would stay away from covering the cement whatyoucallit with small opening mesh, wire, etc...du
