Audi B4 Cabriolet Subwoofer Replacement

Hello All,

I have a 1998 Cabriolet that had those Nokia Active Speakers in the rear and the sub in the ski hatch. I've laid cable and replaced the blown rear speakers which involved liberal use of a dremel but now I want to replace the subwoofer as well.

The sound in my replacement stereo sounds fine with the sub disconnected but as soon as it's connected my speakers make the kind of noise speakers make when there's a bad connection. I'm pretty sure it's the sub causing the issue so want to just resolve it.

Does anyone know what size subwoofer fits in this hole as a straight replacement? I've got an amp and wiring kit sitting here ready to be installed but no sub yet.

Any help would be appreciated!