I have cut the lips of the buckets to fit them into shape.
(Blue line indicates where I have made a cut)
They were overlapping with too much gap, so I trimmed down a little more on 1 quart buckets to make the good width of band like shown in Marmit backpack:
After the rough fitting, the 1 quart buckets were too long, so I have trimmed along the 4oz line of the buckets.
(50ml line pictured. But by testing, it turned out to be needing more trimming than the 50ml line, so the 4oz line was used as final trim line)
I am happy with the shape they make right now. I will glue them together soon and update more here.
Please feel free to give me suggestions!