mounting my new fuel cell

I just got my aluminum fuel cell in todaya nd i want to get some opinions on how i should mount it. Im either thinking about mounting it in the bed on the surface or mounting it from below the bed and cutting a hole in the bed for it to come through. The cell is 12" tall and i would have it about 6" above the surface if i were to mount it under the bed. it fits perfectly between my frame rails and will be located near my tailgate.

I also wanted to ask you guys if it would be smart to isolate the aluminum mounts with rubber gromits to keep it from cracking over time?