Calipers

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.

^^ 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