Tricky question Gilbert. It depends on how it fastened down etc. 1mm is pretty thin, but if it was formed and welded into a tub and bolted in it would work. Or you could make a plywood shape and then attach the sheet to it. If you went for 3mm, it would start to get pretty expensive and harder to work with yourself. But would be self supporting in a sheet, if you follow. So I suppose that 2mm would be about right. You could have a tub shape powder coated which would be very smart. Tread plate has some additional strength because of the raised pattern on it, but it can actually look a bit much in one large sheet, plus it's a devil to clean actually. I really fancy one of the phenolic resin type material myself. Metal sheet is quite harsh and quite damaging to whatever rolls around in there.
C