Installing the fuel cell is not all that stright forward.

The 911 trunk floor is a complex shape without a straight lin eon it.
As such we decided to build a lightweight fram eout of sheet metal.

You do need to move the existing battery support a fair amount and reinforce it.
 

Our frame was made of 16 guage sheetmeal and angle, welded together
with gngles bent and welded in place to follow the floor contour.

Nice fit.  It's held in place with eight 8mm bolts.

Install looked pretty clean once we painted the frame

Ended up taking about 5 hours for two of us to fabricate. and clearance the battery tray (see lower right).
We welded in some angling to reinforce the removed area.