OMG look at her trunk!
How bout a changeable center section? Have a round hole in the main floor piece that's just bigger than the spare wheel well then have 2 different pieces to put in there, just a round flat piece to cover a spare on long trips etc. and a center section that has the storage and square lid in it like yours has, then you can swap the 2 out whenever you want! I think this makes sense. I hope so.