Jan,
I took mine out last year when stripping the headlining in the forecabin, but remembering the thickness is another matter....!
It is certainly not as thick as 12mm, and I recall thinking it was fairly thin. I would guess 8mm or 10mm, but incline to 8mm. There is a huge amount of chamfering where it butts onto the hull sides.
Perhaps one of the owners who lives nearer his/her boat than you and I do would be able to confirm?
John.
275/30 "Cascada"