Our biggest bung is currently 50mm. You can use some cork or wood to fill the hole if the aperture is bigger than that and add our fittings to the top.
These parts are usually put together for customers as you need to switch the reducer round. If it is not assembled for whatever reason you just need to do the following.
Undo the reducer screwed on the top of the bung (see image below).
Screw the side entry tube onto the bung where the reducer came off.
Put the reducer on top of the side entry tube ready for your lampholder.
The side entry tube is now the right way up so the flex can be gripped by the black cord grip above the entry hole in the tube.