At last, time do some carpentry.
For the floorboards I purchased six lengths of Queensland Hoop Pine 3.6m x 250mm x 25mm. Although classed as softwood the timber is dense and weighty and has a beautiful grain, I think it will make perfect floorboards plus the off cuts can be milled up for cleats etc.
As the purchased timbers dimensions were 250mm wide by 25mm thick they required considerable milling.
I cut two lengths a bit over 76.2mm wide on the bench saw and the proceeded to mill down the thickness but unfortunately, my trusty old thicknesser/planer blew up, so it was off to the local hardware store for a new one.
Time to make some dust.
The first two center flooring boards are trimmed, and edge routed and sitting in place around the centerboard case.
The jig from a different angle showing the spacer screwed and glued into the ply. I used two wooden paddle pop sticks glued together to provide the 3 mm gap between the floorboards.
Here is my jig arsenal, consisting of the drill jig, countersunk drill bit, spacer jigs with 3mm nails and two paddle pop sticks glued together to form a 3mm thickness.