I am not clear on what you are looking for. Is this:
A flume to direct runoff toward, or away from, the throat?
A weir/baffle on the bottom of the inlet box?
An energy dissipator at the end of the pipe?
In any event, first check a local precast supplier. If this is a standard product (or even a fairly common custom one), they can almost certainly offer a product at an attractive final cost versus cast-in-place.
Engineering is the practice of the art of science - Steve