Having completed my design it is now time to make up the form that will form the basis for the rib construction.
Having scoured the net to see how other people do it, I eventually plumped for a design based on a central plywood form with removable plywood panels on front and back to allow fitting of the linings.
I started by creating a template out of a piece of 6mm clear acrylic sheet by sticking a full size plan printout to it and band-sawing and then sanding to the plan outline.
Then I used the template to mark out a rough outline of the form on the central 18mm plywood and then routed back to the template. I then used this template to mark and rout the two outer 12mm plywood elements of the form.
Next I cut out the centres of the plywood shapes to reduce weight and provide an anchor for clamping and then cut the notches for the corner posts and end posts.
To get the 120mm rib depth I glued some 1.5″ battens to both sides of the central plywood form and dowelled everything together to ensure alignment.
Finally a few coats of varnish after sanding the edges round to ensure easy removal from the assembled ribs.