This is how you do it.
The picture of the soil pipe below where i put the red that is the groove you cut the same width as your gooseneck.
Now the screw cap remove it from its holder and cut the holder as shown by the red again and then glue this in the end of the pipe both grooves lining up.
Once in place you have your groove and the end cap will screw on.
And the cap for the other end just glue in
You can get all of this from screw fix just type in soil pipe.
Also halfords sell a kit to fix these to your roof rack for about £10
Paul