There is a bracket you need that sits within the red plastic pieces. I tried to find a pic. Mine is mounted below. Pretty sure it’s a Porsche part, mine doesn’t seem to be aftermarket. It has two din slots in it.
I think this is it right here...
https://www.ebay.com/itm/1997-2004-2005-Porsche-911-Boxster-Stereo-Bracket-Support-Mount-99655211502-/182296145521