And finally plucked up the courage and applied drill to fibreglass.
Hole locations seems absolutely perfect.... but there is one other problem... the hollow box section upon which the dog house sits isn't hollow!
The drill bit finally gave up after cutting through 2" in the first hole and 1" in the remaining 3 required on the port hand side simply by becoming too blunt to cut.... fibreglass and hardwood are pretty tough on drills!
So, it appears that i've got 40" to cut through in total on one side and 10" on the other.... that'll take some doing.... and some new drill bits!... a pic to follow.
I have however found a lovely bit of hardwood in a locker below that is perfect for mounting the deck organisers and the clutches, so brought that home to cut, rather than try in the confined location of a boat.