All one bit of carpet, with sound deadening underneath.
No build pics as there is really nothing to it.
Take everything (including all the plastic trim) out move the alarm sounder to one side and use an existing bolt to re fix it,
Spot drill out the welds holding the spare wheel bracket. This gives you a flat floor to go at.
Then chuck a bit of carpet in there and cut it to fit use carpet tape to hold it in place, lap it up the sides under where the plastic trim will go back, and Bobs your mom's brother.
I have put a smaller (Under seat) sub woofer back in the boot by moving the bose amp across slightly to make room.
Its under the shelf thing in spaces usually filled with polystyrene blocks.
Ill get a photo up when it stops raining.