For measuring the thickness of your ribs the diameter of the pegs and the edges of the soundboard. These can be very expensive but this is not necessary, get the cheapest and smallest you can find. Strictly speaking you need a long-reach caliper to measure the thickness of the soundboard all over, however these are expensive from specialist instrument makers suppliers. If you are going to make more than one instrument, this is probably a tool you will have to get or make at some stage. See picture for my own version which I made up in hardboard and then had cast in aluminium. Next to it on the right is the cheap caliper.