I have not built one in ally before, but from doing heaps of repairs on heavy machinery tanks, just a side note hard corners are not very durable, they seem to get affected by vibrations and not handle impacts brilliantly, but it is of course the cheapest design so they make, I fix.
In building an aluminium one, I was thinking of domed ends circlular or rectangle, welding a sheet between them bent around to shape. Welding baffles in to bottom and sides, stitched only but leaving them 2 inch off the top, with a suitable hole/ holes on the bottom of baffles for fuel transfer etc, bending top over and sealing. Thats the plan. I figure the domed ends will be more durable and any hard knocks wont transfer down hard cornered welds. I know the tank I want is only 60 litres but as a paid for prototype for something else, I am comfortable to go above and beyond the qoute to get the design right etc.