I wouldn't use any doughnuts if I were you lol
If the one pictured is the one you are getting then you will need three parts.
First off, a fitting kit to connect the downpipe to the decat part. These kits come with springs, bolts and nuts and are about a tenner from a parts place.
The second is a horseshoe clamp as the centre looks like it slots into the decat so about £2-5 from a parts place.
The third I am not sure on the name but it connects the backbox to the centre, it's not a hoseshoe clamp....there are two kinds of clamp and this is the other kind. It looks like two humpback bridges and will be £2-5 also