If you try and go for a full size floor in one piece I think you will find you won't fit it in
, half the reason they make them in 3 and 4 pieces for other boats.
As for a template, the under side of your inflatable can give you the size you need. Inflate the tubes, turn the boat over you can get the size of the floor from the centre of the seam around the floor. You can start tight and keep removing a bit from the ply until you can get it in.
Minimum pieces you will get away with is two pieces of ply. Make sure you remove all square edges. For the long edges use a hand plane and sand paper or a router with a quarter round cutter in to round edges.
Once you know it fits, plenty of coats of paint or varnish.
Hope this helps.