I made mine from 2mm but I suppose with careful attention to where the brackets are attached for mounting the tanks you could go thinner, especially if you made a few folds to strengthen it up.
I picked 2mm because I hadn't welded Al before and wasn't sure I could manage with anything thinner. Plus I wasn't bothered about weight saving, it was the non-rusting aspect I was interested in. There's still a slight weight reduction, density of steel is 7.85 against 2.6 for Al so 1mm steel=3mm Al. (very roughly)
Brian