Wow thanks for this! I have just uploaded some photos of my hull with part of the deck removed. (pic attached)
The floor on mine was 12mm but again I cant tell if from Halmatic or replaced later..
There does seem to be a lot of support beams so I doubt ill need 18mm anyway esp after seeing yours with 9.
also I notices on mine where something solid was affixed, they had 10mm aluminium plate underneath which was bonded to the underside of the ply and then drilled and tapped.