Sounds like you should consider using what is commonly referred to as a "stop-log" structure. This is often used when flooding fields for waterfowl, but I don't see why it wouldn't work for you.

Basically it's a frame made of metal or concrete that is used in place of an exit pipe. The base is always made of concrete and the vertical legs are often made of channel iron. Treated lumber of the appropriate length and height is slipped into the vertical legs and pressed firmly to the concrete base. The wood will swell and prevent water from going through. So basically, you have a vertical track on each side that guides the lumber and holds it vertical. The concrete provides a flat even base. Sometimes caulk is used if the lumbar is to be in place for a long time and no water seepage is desired.

In order for this to work, you would need to prevent any water from getting to your pipe by building walls on either side of the structure. This would be a permanent modification that would be easy to take care of.

Now if you want to go low-tech, sandbags filled with clay might work.

I also guess that you are using the emergency spillway to regulate overflow, thus the 8" max rise.