I agree that stainless steel would be too heavy, you should be able to go with aluminum and get the strength you need and be light enough to handle. Take a look at the v-hull jon boat that I modified with wood.
http://www.calfishing.com/gallery/Boat-project

I used 1/2" plywood but had supports under it. Without supports it would be a little flimsy so if using plywood I would use 3/4". A little heavy. you can see that a 4x8 sheet left some gaps on the sides. Can't really see them when it's carpeted.
I made a template by using a big piece of cardboard from a shipping box I had. Made some quick measurements to rough it out and then trimmed it in place. Make sure you leave a little boarder around the sides, especially if adding carpet.
You wouldn't have to worry about the deck slidding forward, maybe you could add a lip on the front edge that would fit over the front seat and hold it from sliding backwards. That way you could just lift it out when you want to remove it.
Let me know if I can help more.
Ken