Hi, we've just taken delivery of a new Q2 and we ordered a space saver on-line.
I googled 'Audi Q2 spare wheel', I chose The Wheel Shop that sold an 18" space saver for £125 with free delivery (3-5 day) [I have no connection with this seller so am not suggesting you buy from them].
It came today and we put it in the back.
The foam tray with the jack in it, etc, does do not fully sit down in the centre of the wheel - I think it is designed to sit inside an alloy wheel sitting in the wheel well - as it sits a tad high and so the plastic nut that holds them all down does not reach the centre plastic threaded rod.
We are happy to leave it there as we have the boot floor high as we carry a dog in the back all the time - this leaves plenty of room for the foam tray to sit proud of the space saver wheel.
In the future I may trim the foam down to allow it to sit down further and then I can screw the plastic nut down, but, its not a priority at the moment.
Hope this helps