I too am new to this site and live in the United States.
You could try the manufacturer for the floor or make a template from pieces of cardboard to just fit under the tube, then get high grade marine plywood and cutting to your patterns. My Sea Eagle is 10'-6" and the floor is a 3 piece rigid plastic about 5/8" in thickness, it has two aluminum extrusions per side of different lengths thus overlapping the joints. Not quite sure on how you would lock the seams (2) togather though on plywood but I'm sure someone might chime in with suggestions.
My floor was 599.00 US

to give U an idea.
Good Luck!!