Plywood, polymax kennel flooring, diamond plate, etc. The thing you need fo figure out first is how to support the frame so it doesn't sit on and chafe up your floor. Check out NRS's standing platforms and other fishing frames for rafts and you'll see what I mean.
As far as performance goes, realize that your platform is a white water boat with minimal self-bailing ports. So, expect a lot of drag and added weight from not only a false floor but from intake of water as well. If you're not worried about performance, then stability and safety are your next things to consider. Suspending a floor, or adding a fishing frame is probably your best option. But, again - you are using a white water boat which is purposely designed to stick to the water for ballast, not to be quick.
From my experience, I'd build your standard frame, where instead of thwarts you have an internal frame that comes down and fits snug against walls of tubes and taps floor. Basically instead of a sit on top frame, but what looks like a cataraft frame. Then use whatever material you find easiest to get and works for you (marine grade plywood, aluminum diamond plating, etc) and cut it so it's just a tad away from the walls of the tube. Pretty much inside the dimensions of the tubing of frame.
Onto performance. Do you plan to run heavier whitewater rivers? All the rivers I've ever fished, I don't ever need a sportscar on it (why I rarely ever run my cats anymore). Especially holding water, pulling plugs, etc you don't want a boat that's so super responsive that the wrong oarstroke and your boat is swinging. If you don't plan to pull plugs, or work a slot slowly (I'd rather row a slug that creeps down a slot then row like a madman because my boats moving too damned fast) and plan to run to a hole and get out and fish, go with a more responsive setup. Now when I say slug, I don't mean so damned heavy it won't move. But you want a nice balance. I have over 3 decades running inflatables, I always fall back to my driftboats. But did use a buddies raft setup like I mentioned above and it was damned nice, especially when we were fishing the Calawah.