Think Bubba' Fab, ho-made aluminum fuel cell ...

It wouldn't sit any lower than the bottom of the frame rails (TJ), but could still potentially see a rock or two.

1/8" ?

Or ...

Fab the bottom out of something beefy like 1/4" and the sides and top out of something far thinner/lighter, say 12 gage?

cm "and no, no diamond plate" k

If it is going to be exposed like that and has any potential to see a rock, or even a good branch/root, I'd either make a real good skid plate for it, or make the bottom out of .25"....

and depending on how its mounted, the sides may need to be beefier too. If the bottom of the tank is .25", but the sides are 12 Ga, and the tank is mounted w/ tabs coming off the sides, then even a rather tame hit that would leave the bottom unhurt could rip one of the mounting ears off, thus leaving you w/ a tank not held on very well, and a possible hole in the side where the mount ripped off.

just food for thought...

--Berne