I'm a fan of steel for wheel tubs. They take a beating.
Fabricate from sheet. Add a flange.
Bolt down (with a gasket or sealer, if you want) and seal all edges and corners with automotive seam sealer (caulk type, not the paintable type).
My preference for automotive seam sealer is not shared by some people here; but I think it holds up better to vibration and flexing - something very important for a part of the trailer that will receiver regular impacts - it can be painted, and it can be laid in quite thick to fill big gaps, if necessary.
Remember, it doesn't have to be round. You can bend something up as a hexagon, octagon, or even just a rectangular box.
If you don't have access to a welder, even riveting the thing together would work. It just requires more bends to form flanges for riveting.
It seems like you prefer working with wood. Perhaps the better compromise here is building the wheel tub out of wood and then lining it with sheet metal.