Extend your caliper to measure inside over lips, ridges and other obstructions. You can purchase a long reach digital caliper for $$$ or simply extend your existing caliper. This makes it much easier to accurately measure things that you are drawing in 3D. Simply slip each extender over the caliper outside measure bloops and lock them in the slot already existing on most calipers (also see BTW below). These extenders have a 0.5mm key that locks into these slots.

This extender work on a Harbor Freight 93293 digital caliper that has a curved bloop, I will publish the more popular linear bloop adapter soon.

BTW: for maximum stability you should consider filing a 0.5mm deep slot directly across from each inside top slot most calipers have in each upper bloop.

Thanks