Many woodturners use thickness calipers made from stiff wire to judge the thickness of vessels, but I like soft copper tubing--it's stiffer, less "springy", but still easy to bend. Also, softer, and less likely to scratch the wood. I ended up with a few yards of 3/16" OD thermostat tubing pulled from an old school building, used to make the 7" long calipers shown in the attached pictures. Choose a tubing size that suites the scale of turning you do--for big vessels over 16" deep, 1/2" or 3/4" tubing would be appropriate. Bend to the desired shape, flatten the ends and nip them off at about a 45 degree angle. Use a file and/or sandpaper to round and soften the tips. If you line up the angled ends with each other when setting the calipers, you only need to keep the outer end perpendicular to the vessel's curvature to get an accurate reading at any point.