Hi David,
I had to replace the floor in my 04 Cub Drover t punctured after a blow out punctured the floor. I used from memory 9mm marine ply and vinyl. I coated the underside of the ply with tar paint just to seal it better.
Haven't done the flip over floor so don't know about this one, but i would presume it is about 18mm marine ply with a glued down glass sheet, but I would suggest possibly using a thin alloy sheet.
Hope that helps.