just contact burton services and tell them you need a m6 kit disc and bolts. (of course if you have pre 2011 then you will need m5 kit)
the burton disc fits union perfectly.
There might be a slight bump on the footbed depending on which model union you have but nothing to worry about.
You can also buy the union m6 disc for like $8 if you search around and it uses the same screws and what you have unlike the burton disc m6 kit.