Can a section of PVC (or similar) pipe be forced inside the culvert? You'd have to closely match diameters, then maybe pour concrete around the coupled area to seal leaks.
If you want a positive seal, then it will have to be a wooden gate, rounded to the bottom shape of the culvert. It could be jammed down into one of the culvert grooves. Line the bottom of the wooden gate with some type of rubber - as a gasket. Heavy foam rubber or something similar.
Or, as Diggin It, suggests. Only, use high density expanding foam to seal around the coupled area. With a chunk of PVC extended out into the pond - a sealed gate would be easy to fabricate.
BTW - the pics are a BIG help.
Do you guys think this 15 gallon steel drum would work with what you're suggesting for an insert? The outside diameter is around 14", and I'd be able to simply install or remove the lid to open or close it. And oosik, are you referring to that foam they sell for setting fence posts now? If so, is that something that would remain watertight over time?