For a safety factor of 2-9mm, sf of 3-11mm, sf of 4-13mm
Sent from my SGS4 Active
With the height being only 2' I'd be comfortable making that tank from 12mm, as you would have a safety factor of over 3. With that much width you will need to make sure the stand is evenly supporting the base, you'll have a fairly large area to try get level...
my friend has a 10x4x2 built with 3/4 probably the same