I've used birch plywood laminated with a textured phenolic film. It's used in UK a lot for making trailer floors and flightcases. I had to seal the edges to stop it delaminating over time but it's pretty durable and a lot cheaper than aluminium treadplate of the same strength; I suspect that it's lighter aswell but I don't suppose there's much in it. I used 12mm Buffalo Board but there's a few manufacturers who do it in various thicknesses.