Here is my plan trying to squeeze a lot of the above mentioned.
Using a temporary spare tire mounted on a $100 4 1/4" Fachs, I plan on having a MBZ cooler under the front reinforcement shelf using the factory front holes as inlet and open the two blank ports in the floor as vent. I will redo the floor cover so it seals better all around creating a plenum.
As you can see in the last picture, the floor panel could even fit below the shelf with this compact setup.
This gives me some impact protection, an oil cooler, and a good size trunk.
Initial mockup using my craisglist $15 spare and MBZ cooler.
Click to view attachmentSpare now on Fachs
Click to view attachmentTrunk floor lower height
Click to view attachment