Not sure how your underbelly, with plywood and tarp, was originally put together at the factory but on mine there was just one full sheet of 2mil coroplast. The sheet is about 27' long and was screwed to the frame with metal screws and fender washers.
I had to replace about 16' on mine and bought 4 x 8 sheets in 4mil. Home Depot or other box stores carry it. I also used 1 1/2" aluminum angle across the frame for support and covered the seams with Gorilla Waterproof tape.
Everything attaches to the bottom of the frame with self tapping screws and fender washers. Check out some of the other posts on Coroplast and you will find other fixes. I think I had some photos in mine as well.
2014 Sunset Trail Reserve ST25RB
2013 Toyota Tundra Crewmax Platinum 5.7 Tow Package