I used diamond plate aluminum for mine, although there have been times I wished I'd used steel diamond plate instead just to increase the weight on the back half of the truck.
To build the floor I used a piece of 3/4" marine plywood the same width as the floor, then split it down the middle so I could slip it under the angle rails on the sides, leaving a joint down the middle. Before installation I coated the wood on all sides with fiberglass resin (no cloth just resin) to completely seal the wood. I then rested 48" wide diamond plate on top and bolted it in place. I didn't have any way to get it under the angle rails, so it's centered and just touching the edges of the rails.
For 10 years I had a Gaylord bed cover installed so nobody could see it anyway. It's certainly not show worthy, but for a working truck it's easy and looks nice. The only pictures I have are very poor quality but show how it looks.
I've crawled under and looked at it and other than having 10 years of road dirt on it, the wood looks good as new with no deterioration.