For the pipe itself, I cut a section of an aluminium tube at a slight angle. For the wall, I cut some courses of brickwork off an old tunnel portal.
To create a depression into which I could place the culvert I simply 'dug' a channel in the bank of the small stream and set the culvert in place.
I added more plaster to smooth out the landscape, taking care not to get it on the wall.
Once that was dry, I painted the rocks, added grass and also added water. (I use EnviroTex light.)
Now the water flow into the lake looks much more convincing...