You could use 11 ga or heavier material for the tank.
As to supporting the workpiece, some make a pair of comb-like slat holders which have the slots cut (for the slats), set at a slight angle, maybe 10 degrees so that the slats always are tipped when the workpiece is loaded, and there is no tendency to teeter back and forth.
The slat holders could be 1/4 or 3/8 inch thick, maybe 8-10 inches high. You might use a large angle iron and cut the "teeth" in it. If you choose to buy the slats from a supplier, they can be 1/8 to 1/4 (or maybe thinner), spaced 4-6 inches. Some guys have the slats sheared from sheet stock. Slats are consumed in the cutting process, and sometimes you can tip them upside down to extend the life before scrapping them.
We used to recommend an additive for cutting tanks, but I don't recall the Chicago area manufacturer. You might call a welding supply distributor, one who deals with cutting machines.