Here's a simple thickness caliper I made.

I tried a metal caliper but it scratched the wood. Then, being a cheapskate, i decided to make my own simple blackwood caliper. I was able to leave the tips nice and blunt.  There is a bit of flex, so this is not a precision tool, but it is a very benign, efficient way to measure the top while I do the big work.

This is way more efficient than my big dial caliper. I set it at 10mm, wiggle it all over the top and wherever it touches, I mark with chalk. Then plane till right. Then set at 9mm and repeat. And so on, and so on.

The flex also means I can feel the bumps, and it also helps avoid marking the soft top wood. I set the depth with a neat little graduated wedge I made. This is a really useful tool, too, for setting all sorts of tools including the drill depth when drilling guide holes for hollowing.

Here's a better pic:

I did try a pencil caliper, too, but found on a double bass it was just too fiddly, and scratched the top. I tried fitting chalk instead of a pencil, but that didn't work well either.

 My simple caliper and a piece of freehand chalk is the best so far.