The internal diameter of the flange is 85mm (8.5cm), by internal diameter I mean the size of the hole the air flows through . This is a filter element only I have for sale, it does not come with adapters or tubing.
BTW the box is made out of an old plastic airbox from Renault 21. I can chuck it in for free if you like, it will just hit the bin otherwise. It is a good heatshield and can be ducted for a CAI.