Yup, I went for 1.2mm and they do the job lovely.
But remember that I constructed mine with lots of alloy L section on the joins which adds strength as I don't have the space or tools to cut-n-fold alloy sheet to a good enough standard
So it was sheets of 1.2mm Alloy, Alloy L section, some 1mm cutting discs, tin-snips and rivets. lots and lots of rivets!
In case it's of any interest here are mine during construction on the one small empty space on my garage floor!
And in place:
One thing to mention that I've noticed from driving is that if I did it again I would angle the back wall of the foot well boxes so that it slopes up to the cross member as it can be fiddly getting my heels comfortable when getting in and out.