It depends which swivel you have, the version that I have that is designed to fit over a leisure battery (big central nut facing upwards) has socket headed bolts at the rear and I think nuts with a conical end for the front fitment. I can have a look later in the week if you wish and post some pictures.
As I said, these came with the swivel plate.
The other version of plate which has the big central nut facing downwards uses the same nuts & bolts as those used to secure the VW seat in place.