I ended up using Boardcad free open source design software. It has a function where you can digitise outline and rocker shapes from a couple of photographs.

My camera has a grid function and pitch and roll indicators so it was easy to set everything up square and take the photos.

I used a spirit level as a straight edge and a ruler to check thicknesses and measure concaves and stuff.

It was pretty easy to get a near enough model of my favourite board. There's a few slightly complicated things to sort out but near enough is good enough for now.

While rummaging around the internet I found this picture. It offers a different take on calipers that could be useful. Not sure on where you can buy the measuring probe.

Select to expand quote

Gorgo said..
Not sure on where you can buy the measuring probe.

www.bunnings.com.au/craftright-150mm-digital-caliper_p5660742

^^ I have those and they are actually pretty damn good. Battery life a bit low but veyr consistent measuring.

I love that pic using them as a probe between the two points (the big grey u shape one that Gorgo posted. )
Works backwards so needs maths as u use it I guess (?) but its a very novel solution and I reckon I will build a set just like that