The aluminum will cost a bundle, especially diamondplate. I had this thought too, several years ago. Tried to find aluminum scraps big enough to use, but they were regularly snapped up as soon as they came in the salvage yard.
You would likely have to do a inner structure of wood for it to be sturdy enough.
Most people give up on the idea and just go with a steel diamond plate sheet - which is heavy but great in the winter - or some variation on plywood sheets.
