I haven't done it with a Riken but I did push an old maravia floor into the bottom of a Udisco raft. Didn't need to glue it or anything, just stuffed the floor in there and punched holes around the perimeter, kind of like a Saturn floor.
My opinion: Those boats look pretty narrow, which isn't a problem but having gone through this I'd say the hardest part is finding the right sized floor to mate with the boat. In your case, you're looking specifically for something narrow like a puma but long like a regular raft. That may or may not be an easy find for you. If you end up having to buy a floor then you'd likely have been better off just buying a SB to begin with.
It can work, and work well, but if the boat is 1200 and you still need the floor... I think there was someone selling some used super pumas on here for about 2400 each...those will at least have some resell value.