Yes this is going on the spillway, or runaround area. The guy I talked to just said to make sure the vertical part of the rectangular opening is no more than 2". This man said the horizontal section of the rectangle opening be as wide as possible since he didn't think the fish would turn on their side to pass through the fence.
here is a link that has a good picture. it shows a fence made from solid bars only laying horizontally. There is no vertical bars besides the posts that hold up the fence. In that situation the rectangle openings are 2" tall by 4ft wide.
http://www.tpwd.state.tx.us/landwater/water/habitats/private_water/gcarp.phtml
