While looking for my  callipers which I hadn't used for some time, I came across an old sliding hinge latch? (don't know the proper name) which I cut into two sections and filed the ends smooth, drilled holes for the joining bolt, gave them a slight bend and hey presto, the job was done.
They're low tech but capable of giving an accurate measurement of the total body thickness, which for a wooden violin should be at or close to 64mm.
For a metal body the total thickness is reduced by the difference in thickness between the wooden and metal plates, a difference of approx. 5mm