Both ends have wooden plugs - the bottom ones are maybe an inch or so thick but certainly enough to get a bolt into. I use inserts rather than screws to gain a bit of “traction” and of course they can be dismantled if need be.
The cardboard filler is great as it allowed me to wire the fan and lights internally - all wiring is concealed.
To keep everything nice and consistent I use Ikea pencil plugs to fill the holes for the screw on “feet” on the top part!