At this stage I considered using 3mm paxolin for the main hatch too but I'm sticking to my plan.
Here I think you can see my intention. The deck around the hatch will be doubled up. I thought it was 3mm ply (according to MMM) but it is 1/8th as two are more than 6mm.
So here the shelf will be wider and I'll be using M4 countersunk SS 8 off. Only need access for maintenance{oiling} or repairs
The difference in thickness will be made up with a self adhesive foam strip that will act as a seal too.
The deck is still dry fitted while I am sealing and sanding the ply, first coat on and sanded here.
Also you might have noticed I've omitted the motor connectors as they will not be accessible with decks on.