Is this a joke?
Mechanically fix; screws for cement boards, screws and big washers for foam XPS boards.
Why do you think its a a joke? What do you think this forum is for? LOL
That section is just 130mm wide. Ideally I would put a screw in the left and the right hand side because just a single screw in the middle would potentially pivot.
Apparently I dont need adhesive if its an even surface, but obviously the adhesive would prevent any movement.
The down side with putting a left and right fixing is that its really close to the window reveal and as Im sure you know you cannot drive to close to the edge else the block will split off. esp with spanker dowels.
Im also wondering if there is reason for using 1200 hardies over 2400 foam backer? So sorry Ben if that question isnt valid enough for you. Maybe someone else has some less negative advice?